A few years have been using Premiere Pro.
and I’m starting to migrate some work pro FCP.
I’m there with this problem, I import a video file in Preview and the image looks dark as if with less opacity.
Already converted’ve tried various formats and nothing.
I would post this in the FCPX Techniques forum here https://forums.creativecow.net/fcpxtechnique
There are some very knowledgable and helpful people there who can probably help you.