Caused by 113d91aba8
Code first gathers a list of all vertexgroups on all objects.
Then it iterates the objects to join, and for each of them, puts their
vertexgroups in an index map.
However (by a typo?), it was doing it for the `active mesh` (instead of
the one currently visited) over and over again, resulting in weights
from other objects written to `def_nr` from groups of the `active mesh`.
Now corrected.
Pull Request: https://projects.blender.org/blender/blender/pulls/148259
1de7b4372c