I am having the exact same issue with a survey that is being edited through a dashboard. If deleting the related record would leave the parent feature with no child features, then the delete does not take. The survey form shows that it's deleted and lets me submit changes, but when I load records for that parent feature, the related record is still there. I do have "&version=latest" appended to the survey URL.
Additionally, when I went to the Survey123 site and attempted to delete the related record from there, it deleted my parent record as well, which is a very frustrating result.
GIS Specialist - MO Office of Geospatial Information