The mod is incompatible with Sitting+ (at: https://modrinth.com/mod/sitting+). But I'm sure it can be fixed! The incompatibility only arises in fabric, I used to use both mods on Forge 1.20.1 with no issues but now when you use the mod and you sit the 3d layer detaches from the skin and takes the same pose as the player but behind him or in a different orientation.

Try with the latest versions, also using the compatibility mode. There is no difference between the Fabric and Forge render logic, so more likely either Sitting+ having loader differences or the mod loader themselves.