It is possible to un-assign the action slot from an NLA strip. If then
you enter tweak mode on it and insert keys, a new slot is created on the
Action (so far so good). However, exiting tweak mode did not assign that
slot to the NLA strip, deactivating the animation. This is now solved.
The slot assignment is done when exiting tweak mode because that's
when the whole "sync from assigned Action back to the NLA strip"
happens. Also things like syncing the strip length is done at
tweak-exit, so that seemed like the right place to me to do this too.
Pull Request: https://projects.blender.org/blender/blender/pulls/136601
6d3c621958