Title says it all - when iron golems are on their first damaged texture (after three or four hits from a Netherite sword) their texture is incomplete. It's normally not noticeable, but applying a Glowing effect to the golem causes the holes to become apparent and the texture looks kinda glitchy.

This effect seems to happen less on the second damage stage, and by the final damage stage (golem is about to die) the holes disappear again.

I've attached an image of two golems standing next to each other, one undamaged and the other partially damaged. They have a glowing effect that's been colored red to make it more apparent but this happens no matter what color the glow is.