On Anthelmir's Construct in Oathsworn Pit, I've noticed a possible bug that seems to be present each time I visit. I have only noticed it on the hardmode encounter, but it's possible it was present on the veteran version, but not as consequential.
There is a delay to some spells landing and taking effect.
For example, if I cast Radiating Regeneration, my character will lift his staff to cast the spell, but the beam for the heal only appears about 1 to 1.5 seconds later. This is also noticeable with other spells like Breath of Life where there is a delay to the cast going off. Other healing spells such as Echoing Vigor and Illustrious Healing appear to operate normally.
The delay is noticeable as soon as you enter the boss's room and before the fight is engaged. We've tried turning the hard mode on and off, but that doesn't appear to clear the problem.
The casting delay only manifests itself for me on Anthelmir. For other encounters before and after this boss, spells operate normally. I've encountered the delay on all three of my previous visits to this dungeon.