Name a more iconic couple

Has anyone noticed inconsistency with the 'guardrails' gpt enforces for parody images like this?
I had no problem with this one, and I've see all the great creations from others like Clarkson with all the bands, etc.
But for some of my prompts, it is totally obstinate, like when I tried to get it to replace heath ledger with Kiera knightly in the ' Brokeback Mountain' movie poster....