Part of a good review should be to review how you decided to split your PR into separate commits (does every commit have a single purpose, or did you sneak unrelated changes in a commit? Could the commit message be improved?). Furthermore, sometimes you have to rebase your PR on top of master (for example, because things changed in master too much, or because your PR depends on a patch that was recently accepted in master to actually work), this shouldn't make review comments disappear.