Hi James,
I’ve run into similar issues with C300 Mark III MXF files in Fusion Studio. From what I’ve found, Fusion doesn’t natively support Canon’s XF-AVC format in MXF wrappers very well. One workaround I’ve used is to transcode the footage to ProRes or DNxHR in Resolve or another NLE first, then bring those files into Fusion.
Not ideal, I know but it’s been the only consistent method I’ve found so far. Hopefully, Blackmagic will improve MXF support in future updates.
Cheers.