I’m experiencing the same weirdness as @nayeli …but not on every site.
My main site error404 page works every time, actual or 'activated:
Actual, using an animated gif:
Activated:
https://www.profoundprocess.com/this-is-weird
I get different results on this site:
https://profoundprograms.com/error404 (displays as designed)
https://profoundprograms.com/this-is-weird (gets more strange with each refresh–main image is tiled)
I understand this is in beta, so I don’t see offering this to clients until the bugs are worked out.