Returning script ignored. Bug?

[ano]

We tested the nearly identical situation and Golem did cast returning successfully. Same artifacts, same gems, same script, same situation. I played with PD (from 0 to 40) and gems but always had the same result - golem did cast returning the first turn.
So probably this is a very weird and badly reproducable bug.
That golem was worth about 100 gems total (taking on account forge bonuses and not taking on account forge time and the fact that these artifacts can easily be reforged). And as it happens not first time, I think, it should be investigated by the devs.
I can provide anyone interested with the turn files and description

[Sicaire]

I have had the same "bug" happen to me in an 3.17 MP game with CB 1.21 and single age mod.
I had a golem carrying just the 2 astral gems needed and returning scripted as first action. He got attacked along with 3 or 4 PD by 3 enemy illusionists and ignored the scripted cast returning spell.
I would have said it was because the adverse party was not deemed threatening enough by the AI to use a spell involving a gem cost. The AI commonly ignores gems costing spells in scripts if the enemy army is too small. Problem: the golem got lighted and died with a horribly costly gear of magic items doh...