I'll start by mentioning this is the first time I've made a Github issue so apologies for any mistakes.

Also, I've encountered this issue in the 1.21.1 version and sorry if it has been fixed in newer versions but I thought an issue will be worth it considering how weird it was to reproduce. I have been playing with a modpack and doubt if that had anything to do with it but am willing to provide any info if required.

Here's the issue: If you put an item into the bagpack, then without opening the bagpack you open the inventory again whilst in creative, that specific item you put into that bagpack disappears [Works the same with multiple items]. But if you put some item(s) into the bagpack, then immediately re-open the bagpack, then the items don't disappear.

I know this issue may not bother many, if not most due to it affecting only creative mode but it is an issue nonetheless and felt I should perhaps let it be known.

Have a great day,