Some testing determined that bestiary entries are only unlocked through beating opponents physically with the Bare Edge. Beating them with Lust does not unlock an entry. Not sure if this is intended behavior or not.
I suppose it would technically make more sense to unlock bestiary entries through scanning enemies, but that would raise other issues with unique enemies encountered before acquiring the scanner.
Update - Further testing determined that hitting enemies with at least one Physical attack unlocks the entry. So I'm assuming the check in the code makes the bestiary entry viewable once you've hit the enemy at least once with a physical attack, but since that's all it's checking for, it won't make the entry viewable if you have enough lust magic to one-shot whoever it is you're up against. Suggest making it so the check happens for either the first successful physical hit or the first successful lust magic hit against a new enemy.