I found a workaround for this issue. If you select the default processing profile from the dropdown at the top of the right panel it appears to bring the photo back to its unedited state. You will now see “PP3 Changed” in the history. Click above it on “Photo Loaded” to see the after view, and back on PP3 to see before.
If you aren’t sure what your default profile is, check it (and change it if you want) in Preferences > Image Processing [tab] > Default Processing Profile.