In version 3.?? the hierarchy of individual subgroups could be changed deliberaty. I would like to have this feature back. It was lost while redesigning the groups dialog.
Please don’t open feature request that result from preview versions. We are fully aware that some features are still missing. If I’m not mistaken you already reported that one should be able to change the order of subgroups by drag & drop of groups and this point is tracked as part of https://github.com/JabRef/jabref/issues/2599.