Not saving post meta data

I’ve had some code in my functions.php for a while that’s been working perfectly well until recently. Basically, it adds a meta box to posts of type ‘video’ where you can enter a video URL and when saving it will automatically save the embed URL and the thumbnail URL of that video. This thumbnail is then used throughout the site.

The problem I’m having is that it will now happily find the thumbnail URL but for some reason won’t save it, and I’ve torn apart the rest of my site trying to figure out why this is happening.

Any tips? Here’s the relevant code: https://pastebin.com/x4fDCFEm

Thanks in advance.