FWIW, that was also my experience when Edge added this feature. It was particularly annoying as I did not ever want to use the feature. I guess Firefox wanted to be bug-for-bug compatible? :(
Also I noticed another thing: drag and droping a tab on a folded tab group doesn't work. You have to unfold the tab group then you can drag and drop a tab into it.
It would be great to allow drag and drop of a tab on a folded tab group and if dropped just put the tab at the end of the group, it would make the tab group feature more useful by allowing to quickly select and drop tabs where they should go.
(Yes it's a beautiful feature and probably lots of people want it, just noting that the dragging is confusing. I can't think of a good way to solve the problem that doesn't make it slightly more difficult/nerdy to use.)