Pushing Slime Blocks causes Crash - Concurrent Modification Exception

Tried to run a rail duper on my forge 1.20.1 server, and it completely crashes both singleplayer and multiplayer due to a Zeta Concurrent Modification Exception. Downgrading to the 8/24 build fixed this issue. Thought it might be the duper part but just pushing two slime blocks with a sticky piston crashes in the same way.