It's a known issue on Mi Box. Use SmartYouTubeTV ( https://smartyoutubetv.github.io/ ) instead. It doesn't cause such freeze.
Why is it using avc1 with your box. On my mi box3 it is vp9. I was able to find other video with avc1 and that played fine on my box. Did you enabled something in testtube? https://www.youtube.com/testtube
Or is it a hdr content?
It's a known issue on Mi Box. Use SmartYouTubeTV ( https://smartyoutubetv.github.io/ ) instead. It doesn't cause such freeze.