On reboot MB will load the DB but, I think, only the common displayed fields are saved there.
EG: I do not think <lyrics> are stored in the DB and MB will load this on demand. Like when viewing in the tag editor or when playing a song.
This makes since from a practical standpoint. Why duplicate the lyrics by also storing it in the DB when its easy to get when you need it?
Of course, if MB does need to get the lyrics, you might as well (temporarily), store them to reduce the number of file reads.
That being said, I am not sure if it is really a bug. You are asking for field data which is populated on demand. And on reboot there has been no demand yet.
This is implied by <lyrics> not being available as a displayed field.
I wonder what the extra load would be on large libraries if MB needed to pull in all lyrics at load time...
If it isn't that bad, perhaps this would make a good feature request. I cant think of a way to test this though.