I've done a load of troubleshooting this evening as well as earlier today.

I'm struggling to pin this down to any specific issue, I've removed IPV6 support from a couple of our nodes internally as we do see some not ideal behaviour with some of our IPV6 connections but that doesn't seem to have helped.

I've made changes to our LB and Web server configuration to try to ensure there isn't any weird behaviours there.

I've confirmed the network bandwidth between our front end load balancer server and our back end web apps where media is uploaded to before it goes to our media object storage is good.

I've also for one of the Universeodon hosts tried switching from ImageMagick to libvips as I know that's the direction Mastodon is moving towards in the longer term but it doesn't seem to have helped.

I'm struggling to reproduce the issue and am unable to find any actual servers side errors so far which is making it all the more difficult to figure out.