Clarification on NPR foreign stock capital gains laws and the need to file a 修正申告/期限後申告 (revised tax return/post-deadline tax return)

Short version:
I may have to a file revised tax return/post-deadline tax return 修正申告/期限後申告 for 2024 and 2025, due to not realizing that capital gains on foreign stocks obtained after I became a tax resident are taxable on SALE, not on remittance to Japan, even if I was and still am a tax non-permanent resident 非永住者.

Long version:
My main concern is perhaps a misunderstanding of when do I have to report capital gains on foreign (specifically US) stock investment capital gains, and if I am subject to said capital gains.

Here is my background

• Moved to Japan in March 2023 on a student visa, language school (my intent was "trialing" out Japan, I may have still been a tax non-resident?)

• Started working for a Japanese company and received an HSP visa in March 2024

• Been doing American stock trading since January 2024 (albeit quite inactively)

• Have realized around ~$100 capital gains in 2024 and ~$600 in 2025

• Have NOT remitted any of the capital gains proceeds to Japan (it's stayed in America)

Therefore according to my understanding,

• I became a tax non-permanent resident 非永住者 in March 2024

• I will become a tax permanent resident 永住者 in March 2029
  (Potentially up to interpretation, but it could potentially be interpreted as March 2023 and 2028 for these months, but largely irrelevant to the scope of my question)

Here is my situation:
I do understand the overall US-Japan tax agreement where you would have to file both countries' taxes and then you pay the greater one (and you get tax credit in the lesser one).

I was operating under the assumption that as a tax non-permanent resident, I am only taxed on capital gains by Japan the moment I remit the money to Japan, and that any capital gains money from America is basically not counted by Japan.

This is what you would think if you take the Japanese legal term "foreign-sourced income" 国外源泉所得 literally. I had asked some Japanese peers with accounting knowledge, and also went to my local tax office 税務署 to clarify, and they said the same thing. So initially, I didn't worry about filing these and proceeded on my merry way.

However, the more I actually research the letter of the law, the more I'm convinced that foreign-sourced income 国外源泉所得, is not necessarily "income that is from the source of a foreign country", but rather a specific set of 17 items as defined by 所得税法 第95条 外国税額控除
https://www.zeiken.co.jp/hourei/HHTOK000000/95.html

One would think this line means foreign stock capital gains are considered foreign-sourced income…

三 国外にある資産の譲渡により生ずる所得として政令で定めるもの

3 – Income arising from the transfer of assets located outside Japan, as specified by Cabinet Order

But if you actually go and read the list of Cabinet Order specified assets here
https://www.zeiken.co.jp/hourei/HHTOK000010/225-4.html

1. Foreign real estate
2. Rights over foreign real estate, foreign mining rights, or foreign quarrying rights
3. Foreign standing timber
4. Shares/equity interests in a foreign corporation, where the holder owns at least a prescribed percentage of the total issued shares/equity, and the country where that corporation is headquartered taxes the gain on transfer
5. Shares of real-estate-related corporations (including equity interests and investment units under the Investment Trust and Investment Corporation Act Art. 2(14))
6. Shares of a foreign golf course-owning/operating corporation, where holding the share is a requirement for preferential continued use of the course
7. Rights to use foreign golf courses or other similar facilities

You will find that foreign stock capital gains are not included.
And none of these 17 items include capital gains from foreign stocks either.

Therefore, capital gains from foreign stocks are essentially treated the same as Japan-domestic stocks (which makes the semantics of「国外源泉」"foreign-sourced", essentially misleading and cannot be taken at face value).

So the first misconception is that foreign stocks -> "foreign-sourced" income per Japan. Foreign stocks capital gain is considered domestic income per Japanese law.

Now an even TRICKIER part of this situation is there was an amendment to the law in 2017, which defined what tax non-permanent residents are taxed on.
https://www.zeiken.co.jp/hourei/HHTOK000010/17.html

u/starkimpossibility explained the relevant part of this law in this post:
https://old.reddit.com/r/JapanFinance/comments/11tixy2/capital_gains_tax_on_foreign_stock_as_a/jcjdmpu/

taxation to the extent of remittance applies to the sale of securities that were either:

• purchased more than 10 years before they were sold;

• purchased before you became a Japanese tax resident; or

• purchased before April 1, 2017.

This amendment states that capital gains from foreign stock obtained prior to becoming a Japanese tax resident are considered foreign-sourced income, and therefore only taxed on remittance (as opposed to on sale)

Since in my situation, I purchased all of these stocks and made capital gains on them AFTER I became a tax resident in Japan (even as an NPR), that means these capital gains are NOT considered foreign-sourced income, and therefore I was subject to tax on them at the point of SALE, even if I never remitted the money to Japan.

Is my interpretation correct? And if so, does this mean I will have to file a revised tax return for 2024 and 2025 to rectify the misunderstanding?

I am not opposed to paying a measly few tens of thousands of yen in Japanese taxes, but I would like to do things correctly.

Also I am more actively investing now and already factored in the fact that I'll have to pay capital gains tax on any sales (which means I should be minimal with this), but the clarification of whether this taxation happens on SALE or on REMITTANCE TO JAPAN is highly important.

Regarding revised tax return 修正申告 vs post-deadline tax return 期限後申告…

For the 2024 year, I filed my own tax return because I had a part-time job in early 2024 for a few months. Whereas my employer filed my year-end adjustment 年末調整 for me for 2025 since I (thought I) had a fairly normal situation and didn't need a tax return.

If I do need to amend my tax reporting, for 2024 will it be a revised tax return, and 2025 will be post-deadline?