Is it normal here for people to take your seat at the Cinema (when you’re a bit late)?

I love movies and go to the cinema a couple times a week.
I usually plan what I'm going to see a week or two before so I can get the best seat.

It has happened now 3 times over a couple months that I was a little bit late (as in a couple minutes, lights are closed but ads are still rolling, movie not started yet)

Each of those times someone took my spot leaving me in the awkward position of having to guess what other seat would be free. Once I picked a free seat and after 2 mins another guy came to seat there and I gestured I'd leave, but he declined and went to hunt for another seat himself. This being Japan, these moments make me feel very nervous, as I know I'm "disrupting" the atmosphere for other people (which rules out confronting the seat-thief ahah).

I know the solution is to simply come on time (and that's on me), but I wouldn't expect this to happen in Japan, where people are so law abiding.

Is it possible there is a rule that states that once lights go off you loose your seat or something, that i'm not aware of?

Or maybe it was just a streak of unfortunate events.

Btw I'm assuming the seat-thieves were Japanese but I don't know for sure. Also this happened in Tokyo, mostly in independent cinemas.

No hate, just curious!

EDIT: Why not just tell them to move? It's hard to describe, but imagine the lights are off, the movie is about to start. You gotta walk in front of people, squeeze yourself between 5-10 taken seats, use your torchlight to check which is the right seat, and then confront the thief (with my n4!). Somehow I found it too stressfull\disruptive in those occasions, so to I avoid it I just went to the front where nobody was sitting and picked the best I could find. Maybe I'm too self conscious, I'll try to do better.