I figured this out myself and wanted to share in case it happens to anyone else. It definitely isn't my software, which is fully up to date. So I thought I would try converting the files to another video type to see if that made a difference. It turns out the issue is that some 4K videos have an "uneditable" colorsource. So what I had to do was convert my files from colorsource bt.2020 to bt.709, which you should be able to do with any decent conversion program. The files got bigger, but they look bright and clear again.