If you try to add a cell into an existing group via Ctrl-C Ctrl-V, the original group breaks up in a non-reparable way (in the Manual grouping regime). The cells are neither grouped anymore nor ungrouped. An attempt to just group everything together produces a beep. Only deleting inserted cell helps.
Version 126.96.36.199 of Mathematica, but was present in previous versions as far as I remember.
See the picture before and after Ctrl-V
Yes, acknowledged as a bug by WRI on July 30 (any new cell pasting into a CellGroup mangles the grouping). Currently, I deal with it by saving the notebook, closing, opening and then manually grouping.
Thank you for reporting, this is indeed annoying behavior. And it is good to know that WRI also considers this as a bug.