The issue was that geometry nodes was run on the original curves, and set a pointer to an evaluated material id on it. The fix is to not mix up original and evaluated data by making sure that geometry nodes does not modify the original data.
The issue was that geometry nodes was run on the original curves, and set a pointer to an evaluated material id on it. The fix is to not mix up original and evaluated data by making sure that geometry nodes does not modify the original data.