Videos exported from Photos app weird colour or don't play properly in Quicktime

Hi!

I am trying to export some videos from the Photos app on my MacBook Air running Catalina. I have been able to do this previously, but now when I try to do it I get one of two results:

(1) If I export the video, including any edits, then it comes out weirdly coloured - a strange sepia, with a varying tint. This colour palette shows up in Quicktime and VLC and also if I re-import the video back into Photos.

(2) If I export the original video, then the colours are correct, but in Quicktime the playback is very choppy, whereas in VLC it is smooth.

I've tried this with multiple videos. All these videos play completely fine in the Photos app on my laptop. They also play completely fine in the photos app on my phone.

I have tried multiple ways of exporting the videos.

I have exported from the Photos app on my phone to Dropbox, and the video plays fine in the Dropbox app on my phone and in the Dropbox web interface, but if I download it to my laptop (whether via the Dropbox website, or the Dropbox folder in Finder), it's choppy again.

I have shared the video from my phone using an iCloud sharing link. I can open this in an incognito browser window on my laptop, and the video plays fine in the web browser, but is choppy when I download it. Same situation if I log into my iCloud and open it up in the web version of the Photos app.

I have saved the file from the Photos app on my phone to my laptop Desktop using the 'Save to Files' (ie using iCloud Drive). Again, choppy on the desktop.

The only way that is currently working is if I Airdrop the video from the Photos app on my phone to my laptop, and then it opens fine.

Seems like something weird has happened to my Quicktime.... or at least that would maybe explain why there are these videos it can't play but VLC can.... but does that explain why videos exported straight out of the Photos app have this strange sepia colouring?

Any idea what is going on/what more troubleshooting I can do/how can I fix this?

Thanks!

Andrew