During Thunderbolts Mountain it happened twice, I’m absolutely sure it didn’t drained the enemy’s strongest color 
even thought it generated yellow properly.
Will keep an eye on this and will post if I find something else.
I’ve noticed several other abilities’ AP draining effects sometimes not working correctly as well.
Seems to be an issue affecting all of them, but with no easily discerned rhyme or reason to it.
There was an old issue a while back where any ability that drained Yellow AP failed to do so. The easiest way to check would be to fire off IM40’s Red or Blue.
Could this be another issue where it’s draining a different color than expected, in the case of a tie?