Simple issue I noticed first night in Nathria, I configured my nameplates to turn green when a mob is attacking another marked tank, however the color doesn't seem to change when this is the case - I was using version 9.0.2.23

Somethings you could check.
Is the option Highlight Mobs on Off-Tanks enabled?
Is your threat mode set to Auto or Tank?
And lastly, was the other tank marked as a tank in the raid group?

Those are the only reasons, outside of it being broken with Shadowlands, for why it wouldn't work. But I'll check if it is working as intended the next time I'm in a raid.

Is the option Highlight Mobs on Off-Tanks enabled?

I was missing this one - I did a fresh UI before shadowlands and must have missed this option and it fixed my issue, thanks!!!