I noticed that Art of Heat Up wasn't making Aura's Heat Up II a drastic buff. On a hunch I used the cheat console to make her MATK greater than 50 again and it worked properly.

The text of the skill itself doesn't specify a required minimum stat to function-- the book has one to read it, but requirements to learn a skill aren't usually the same as requirements to continue benefiting from them-- for example, you don't lose access to rampage III if your attack drops below 80.

With options like magic threads to dump MATK after learning the spell, in addition to the usual way of dropping stats like gaining a stage of the Cursed condition, if this is the intended behavior I think the text of the skill entry itself should be modified