For what it is worth, it behaves the exact same way on all the instances I tried it on. sopuli, ml, world, and ani.social all will download the file when clicking the icon next to the post title. The embedded .mov file in the post body will play in browser for me in all cases.
I am using a vanilla firefox on PopOS for reference.
Yep, that was it. The file downloaded at the /c/ level after checking that box for this instance and didn't for the ml instance. I tried to check how the behavior is different on the world instance where I have an account, but saw that option isn't there since they are still running lemmy 0.18. So, it seems like that is a recently added option, perhaps some lemmy version differences that aren't visible just through the version number at the bottom of the page?
Alternatively, the only other thing I could think of is some weird webserver setting differences. I only know enough about nginx configs to get myself in trouble, so I can't even begin to try to diagnose it though.
That is weird. On .ml and .world, clicking the thumbnail opens the streamable link in a new tab for me, but on ani.social and sopuli.xyz, the video downloads.
Tested on Firefox and Chromium on Debian so it might not be a browser issue. Same behavior with Chromium on Windows so it's not an OS thing. I don't know what's causing it to be honest.