Recorded lots of footage in 4K 30p and 1080p HEVC and have approx 5-6 pixelated glitches when the 10 minute project is exported. Have tried every combination of deleting render files, optimising media, re-exporting, with inconsistent results and no actual clean export. I read somewhere that I should have shot in ‘most compatible’ mode. Not much I can do about that now but, if this is true, why does Apple not warn users that it means compatibility with an Apple product!