This happens for every modded seat.
I'm not sure if this is fixable without just redoing how offset works when sitting

Easily fixable by removing the last else statement from the public void updatePositionOffset(Entity player, Vec3 defValue) function in MinecraftWrapper at the check for vehicles.

Removing this makes the earlier defined realYaw value used instead of the riden vehicle, which for most modded seats doesn't change direction.

I have fixed this locally, just lacking permissions to create a pull request.

This has been resolved, together with other sitting rotation issues.