I do apologize that I can only offer theory behind the suggestion. The reason is that I do not use Bazzite and I am aware there are differences.
And, I have not used Drop Box myself for over 10 years. So I would not be able to test any theories I might come up with.
But I am convinced that it has something to do with / being of fs type composefs; which the Drop Box flatpak does not recognize.
Oh and did you notice the date on the symlink /home (repeated below)? composefs is not a full featured filesystem. It was introduced as a workaround to a serious defect in btrfs. It is that same defect why we all (users of Universal Blue offerings):
- are stuck on an old kernel version
- will be moving away from btrfs some time this year
$ ls -ld /home
lrwxrwxrwx. 1 root root 8 Jan 1 1970 /home -> var/home
This is an observable symptom of at least part of the problem.
So at a higher level - my suggestion was to configure the Drop Box flatpak to use /var/home (whose fs type it DOES understand) instead of /home (whose fs type it does not understand).
I hope that makes sense.