I think this is happening because slates don't extend FaceAttachedHorizontalDirectionalBlock, so Create doesn't know it needs to handle them differently.

Create seems to have methods to register blocks so they'll be handled properly - I guess Hex needs to call these if Create is installed? There is a create:brittle block tag, but without handling isBlockAttachedTowards, the slates just always pop off and can't be attached to contraptions at all.

https://github.com/Fabricators-of-Create/Create/blob/mc1.19/fabric/dev/src/main/java/com/simibubi/create/content/contraptions/BlockMovementChecks.java#L87-L93

Issue still persists in Forge 47.2.0 Hex casting pre-634

I'm not sure if this bug is still present in the 1.20.1 prereleases, though I'd guess it is.