I can’t seem to reproduce this with suggestions to myself, but I have tested this on the open suggestions that have been shared with me: Forking from the compare view of a suggestion works as usual, but if I navigate to the forked notebook and then click the Fork button at the top of the page, I get the red error message “Oops, forking failed. Try again?”
I was lucky and managed to avoid losing any work, but note that this means that edits to the target notebook in the non-compare view can’t be (easily) saved.
After looking at the network requests, it looks like the POST to api.observablehq.com/documents/.../fork sends a full "comments" array in the non-compare view case (broken) but not in the compare view case (working).
When forking suggestions with comments to myself from the non-compare view (working), the POST sends "comments":[].