This is a bug that seems to have persisted for long based on the old reports of the same thing, but the freshest is from 5 years ago, so I thought to write it.
Casting Flame Weapon on gloves will overwrite the Slashing damage bonus that Sorcerer perk Scion grants. It can be fixed by resetting your perk bonuses every time after casting Flame Weapon, but that is a bit of a nuisance.
To expand on this:
Any damage bonus on gloves overrides any other. If you cast magic weapon and then flame weapon, you lose the +1 from magic weapon. If you cast flame weapon THEN greater magic weapon at 10, you lose flame weapon and gain the +3 from GMW, etc.