Thanks, good thoughts.
If anyone sees the splitter doing that again, check if it persists across a save/load, and if it does send me the middenmoon.json.
There are two code paths that rebuild belt segments: one slow path at load time that scans everything, and a fast path during play that only touches the subset of belts affected by a placement or removal. They share some code, but not all. Either answer will narrow the search for the bug.