Ok, this is very strange. I did some more messing around and it seems to only be affecting one project. This is SD footage shot years ago and captured with an SD video camera (can’t remember more details, it was a while ago). Other projects I have, both SD and HD are not exhibiting this problem. It’s not a huge deal since now I know it’s isolated, but it’s still very strange.
Ok, one more thing now. In the affected project, if I choose a clip, reveal in finder, and drag said clip into the browser again, it works properly. Could the master clips have become corrupted somehow?