Need Insights : Japanese work culture

So it hasn't been long since am in Japan and am learning the culture and rules here, so I understand punctuality is very important and I always try to be on time and somehow I have never been late to work
So today I went just before the chime rang , usually after reaching office I go and buy something to drink from the vending machine , when I went today and came back one of my coworker blocked me and told me am "always" going to buy drink after chime and it みんな気にしている.But I never consciously go after the chime at least I don't think am doing it "always". I do see people going to buy drinks after the chime not immediately though and I didn't know even if I do if it is something of a big deal , help me understand if am missing some cultural context here ? When I consulted two other coworkers one of them said they don't think I do it "always' but it is generally good to not move around once the chime rings , while the other one said there are no rules like that and i can go around even if the chime rings , I know it's not a big thing but am trying very hard to follow the rules here and even small comments like these specially in front of others make me very sad.

by Diligent_Cod_9340