I think so. After more testing with this tag, I can say that that is in fact, what's happening.
Though I'd like more clarity as the naming of this tag can be pretty misleading.
Again, this proves my theory of Mojang not fixing ANY bugs if they don't affect vanilla gameplay. You guys can go and raid people's bug reports with this theory. Pretty sure we can find out which resolution goes to which bug now ourselves.
Also, this is not caused by 1 item having a stack count of over 64. Not just that, multiple items with a valid stack size also cause this bug to happen. The size only needs to exceed 64.
Found that while I was testing my Bigger Bundles datapack. It has been bugging me for so long.
oh nooooooo why won't they fix this?
https://www.youtube.com/watch?v=-bPu7halXyA&start=27
I guess they don't like custom bundles ... I personally loved this project.
Found the solution to the previous problem.
Nothing gamebreaking, that problem was not even a bug.
This one is though. So I just killed 2 birds with 1 stone by overwriting the last one with this one which is actually a bug.
Man I suck at making good reports lol. Need to be better at it soon.
Hope this is the only Realms bug. I haven't used Realms in years now.