Running this on my own modpack, and the block placement works correctly in singleplayer but for the server the placement auto-reverts to a vanilla placement.

For example, placing a slab against a wall so it is vertical has the slab appear as it should for a moment before swapping to either a vanilla top or bottom slab.

Sounds like on the server end the placement logic isn't running (blocks are placed on client first, then the server notifies the client of a blockstate change so if the placement logic is different between the two it's going to appear like it switches after a delay)

Please note what game version and mod loader you are using, and list any mods on the server and the client (separate lists please!)

Running on 1.20.1 using Fabric 0.15.7

Mod list for server:
Server Mod List.txt

Mod list for client:
Client Mod List.txt

Okay, just restarted the server and it seems fixed now. Strange that it didn't change when the server restarted earlier though...
Anyway, it seems to be fixed, might have just needed a couple restarts for some reason. Thanks anyway!!