Bug introduced in 12.0 or earlier and persisting through 13.1 or later
The bug is absent in Mathematica 8.0.4
I am not sure whether this has been addressed somewhere already, I couldn't find any comment.
I suffer from an annoying bug: when I copy and then paste a cell between grouped cells (forming eg a chapter), the grouping is destroyed (creating a new empty cell is fine). If there are several levels of nested groupings, all these groupings are mangled. This pretty quickly messes up any well-structured notebook.
This seems new for the Mathematica 12, 64 bit frontend for MacOS. However a precursor exists already for Mathematica 11: this happens eg if one deletes a cell and wants to reinstate it via undo.
Is there any way to get around this?
Alas, even in the new release 12.1.1 which supposedly has 1000 bug fixes, that one wasn't fixed. So it seems that the interest in fixing this is basically zero.
This is very unfortunate as the situation as it is now is so unworkable that it prevents me to upgrade to Mma 12, which in turn is a pre-condition to upgrade to macos Catalina. So all is stalled because of this.
I just checked the new Mma 12.2: even about 1.5 years after I submitted a bug report, and after 1000+ alleged bug fixes in 12.1, this one still wasn't fixed. I guess they won't do it. I am just wondering why they don't remove the "manual grouping" option for cells for good, as it is just not useable.
So, also in 12.3 this bug remains. After 1000+ bug fixes last year, and 1000+ bug fixes this year, this important one still has not been fixed.