I am quite unsure about what to do, if my answer got accepted, but does not (in my opinion) answer the actual problem, but the original question.
In comments to my answer the actual problem got clear, so I provided a link to another question with an accepted answer, which then (I think) finally solved the issue.
To be more specific, I am talking about this question How to delete a polygon created with "Add polygon Feature" - QGIS with this answer https://gis.stackexchange.com/a/342835/107424
Shall I delete my answer? Or edit it to include the link from my comment? Mark the question as duplicate?