The same thing happens with exploding demons, but in their case, it's more of a feature than a bug. Because of their size, their absence is not noticeable, but it also doesn't interfere with the effect.
Unfortunately, the situation is worse with golems. They are larger, and the fact that they disappear for a second is, to put it mildly, noticeable.
In general, the demon has two spell animations, but only the second one is played in the game during the explosion.
If anyone has any ideas on how to fix the problem, I'd be happy to hear them. We could, in principle, make the explosion effect larger, but I suspect that it will look bad.
Or, we'll have to give up on this ability (possibly in favor of liquid flame).
By the way, does anyone know the name of the unused in-game ability that causes an explosion after death?