Hard to tell... (what exactly it is I was doing)
Basically, I'm doing the same thing (for now) all of the time: tagging, embedding lyrics/artwork, switching between libraries (by closing and reopening MB), making backups with virtual devices, and rescanning (all libraries) every now and then...
And then, all of a sudden, I notice that I can't rescan this or that library. And I work around it with 'send to'.
A few days later: no problem again... So I don't know... (okay, must be disappointing as an answer, hard to replicate... but I really wouldn't know...)
Yet your 'trick' seems to work!