Swiftcast Holy not instant in Garuda HN?

[Inflorescence]

Today I ran Garuda HM a few times, and was trying out Holy-ing the plumes. But while I used Swiftcast, it seemed like it still took the 5 second cast time. No casting bar appeared, but my character started hovering around in the casting animation, like it was lagging (but I wasn't).

A few things I noticed:
-Before Garuda is engaged, the skill works normally.
-If I start casting a spell during Holy's "cast time," when Holy goes off, it'll interrupt my cast.
-The bug happened about 6 times, but with 100% certainty while in the fight.
-Unfortunately, I was unable to see if the Holy counted the damage where it finally casted, or where I initially swiftcasted it. I want to say it didn't cast until after the 5 seconds, when the animation appeared.