I've found this bug with several photos.
Start with a photo edited in a previous version of LR. In develop module, there's a list of commands in the history on the left.
If I click anywhere in the history, all edits above where I have clicked disappear and replaced with "Post-Crop Vignette Style: Paint Overlay". I have no vignette applied.
"Undo" doesn't then work: the edits are irretrievably lost.
Has anyone else seen this? Any suggestions?