Why is this happening? (Clip looks faded when added to timeline, no effects added)

Does anybody know why this is happening? Some of the clips end up looking faded once added to the timeline, but they look normal when in the project window. It seems to be completely random and it's happened to 4 or 5 different clips so far. There are no effects added to the clips, and I've tried starting a new project with the same clips and they look perfectly normal in the new project. I've also tried deleting the affected clips and re-imported them, and they still look faded. I've tried deleting the files completely and re-importing straight from my device (iPhone) again, and they still look faded in this particular timeline. 

I've managed to make them look slightly normal by using Lumetri edits as I don't want to start my whole video over in a new project... But if anybody could tell me why it's happening that would be great!