When I first setup my phone after I got it (which was with the China stable ROM, I'm pretty sure), Mi's version of motion photos was working in their own Gallery app, so I assume they are using their own technology to get that working, meaning it's probably incompatible with Google's Photos app.
I tried flashing the China stable again yesterday through TWRP, but after doing so, my phone didn't want to boot into anything other than TWRP anymore, and whilst I'm not able to do any backups through that, I really don't wanna wipe all partitions again...
I was not expecting Xiaomis version to be compatible with Googles, I was just thinking that if Xiaomi did indeed try to "copy" Google's version, they might have set up a similar system where the "motion" is enabled through some other more sophisticated app than the ordinary Gallery.