I've seen that too. If you double click on the Design Tree Splitter (highlighted in yellow in the screencapture) you'll get two panes and the Rollback Bar will be visible in both.
It doesn't happen with all files but I have seen it occasionally. It's obviously a bug but I don't know how to tell if it's one they know about or are working on. I think it's been there for at least several versions.
SWX 2018 SP4.0
UPDATE: It seems to only be a problem when the contents of the Feature Manager Tree extend beyond the bottom of the screen. If I group items into folders to "shorten" the Tree I can see the Rollback Bar again. It's as if SWX can't scroll far enough beyond the last item (which is there btw) to show the Bar.