Curve become straight error

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.

When I try to delete the bottom node, it become like this.

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.

Which exact build? Make sure you run the latest beta. Go to Glyphs Mini > Preferences > Updates, activate both checkboxes and press the Update button.