Currently suggestedParentNodeTypes is only checked during isolateAndUnwrap, which means if you try to convert a paragraph to a heading inside a list it will unwrap (the primary use case), however we should protect against other actions that would lead to headings being inserted into lists, such as copy & paste, or naive API calls (such as SurfaceFragment#insert).
parentNodeTypes is enforced during document#fixupInsertion, which is run on every transactionBuilder and is much more robust.