This behaviour was found when testing for ICE-9267, and pre-dates those changes.

messagePersistence = true && required=true
Upload successfully or other failure, and that message will show. Then do a non-multipart/non-uploadable execute, and required will replace the original message, even though no upload was even attempted. Or do a multipart upload with no file selected, and now it will change to the required message, as expected. The difference being that there's no existing differentiation from multipart and non-multipart submits.

messagePersistence = true && required=false
Upload successfully or other failure, and that message will show. Then do any execute without an upload, and the same message will continue showing, as expected.

messagePersistence = false && required=true
Upload successfully or other failure, and that message will show for one lifecycle. Any execute without an upload will not show the required message, because there is no determination between multipart submits without file selection versus non-multipart/non-upload executes that are just subsequent executes that would require messagePersistence to be on.

messagePersistence = false && required=false
Upload successfully or other failure, and that message will show for one lifecycle.