Video having artefects/low resolution after exporting

Hi all,

Please help! I'm editing some videos in Lightroom and whilst editing the resolution looks fine, but after exporting (as 'MP4') the resulting file is much lower resolution with frames with bad artefacts/pixelation.

I can't see anywhere to check the export resolution or if anything could be causing it...

I usually use Lightroom Classic but wanted to add grain so used Adobe Lightroom on this occasion but I'm having this issue that I've never had on the other version.