I tried to flip vertical and then it suddenly became like this. My curve becomes a straight line but with no handle. So I try to undo but it’s not working (like when you open the file for the first time, can’t undo). It happens randomly sometimes. This is glyphs mini btw.
You somehow messed up the node structure. I would try to save the file (under a new name) and re-open it. This might save it. If not, either see if you can fix it in the file or find a version in an older file.