Seems like a bug.
As it should provide a +10 to the roll and only pops up to ask you to use it in circumstances where it would cause an attack to hit (I.e. If you needed more than 10 points to your attack roll to hit, it wouldn't bother asking you to use it)
Even the only possible reason it might fail doesn't make sense on multiple levels - That being similar effects not stacking (I.e. A +1 Greatsword wouldn't get any bonus from Magic Weapon as it already has a +1 to damage and attack rolls as a magical bonus from the +1 enchantment)
Given that such a thing doesn't exist in 5e/BG3. Nor would you have another "Divine" bonus to your attack roll. Then even if you did, the highest bonus would always be used so even if your +5 bonus was the one being replaced you'd still get up to 13 attack from the +10 which would still be a hit.
