Mac Pro (2013) & Canon footage graphics artefacts / glitches - any suggestions?

Hi all,

Wonder if anyone could suggest a cause for the graphics problems seen in the image below? It's Canon c300 mk1 MXF footage (50Mbits x 1080P) on FCPX 10.4.4 on a MacPro 6,1 (late 2013) running Mojave 10.4.3.

It shows up whether ingested straight from the card or via an HD backup and in transcoded & proxy versions. It's not always at the top edge of frame but it often displays as a similarly distorted horizontal line.

Weirdly, I have a MBP 2016 running the same version of Mojave & FCPX where the problem doesn't occur (below) on exactly the same piece of footage.

Makes me wonder if the problem is another piece of software or (maybe?) a graphics card fault. Error only present on recent C300 footage not stuff that I've ingested previously, possibly since the FCPX 10.4.4 update (though I'm not 100% sure it's connected as it didn't get spotted until some time afterwards).

All suggestions welcome.