It seems that this was a bug in our upgrader.
The WYSIWYG widget was indeed limited to be used in the pages app in the past, but we changed this 2018 for IPS 4.3.4 (geez, time flies:D ) , but due to a bug in our upgrader, the restriction was never changed, so all instances which were installed before we made the change were unfortunatly not able to use the widget outside of the pages app, while new installations were, hence the problem with reproducing it.
I have fixed this now for an upcoming release 🙂