Apparently, after a PR has been merged (effectively closing it), it no longer changes when the branch upon which it is based changes. This seems reasonable. Otherwise the PR would not be a permanent record.
I discovered this behavior after merging an earlier version of the ekr-vim branch into devel. After that, I had to create a second PR to record the later changes.
Imo, this behavior is not only reasonable, but useful. It's a way of "allocating" diffs to different phases of an issue, thereby reducing the diff's sizes.
All comments welcome.
Edward