As I mentioned in Issue #181 (which was closed, despite still being a problem), the Target of Target frequently disappears. I identified it in SoD; however, I am now playing Anniversary Edition and am having the same issue. It's definitely SUI. You will have a mob targeted and suddenly, its current target no longer shows...the frame disappears completely. It doesn't resolve until you do a /reload.

Are you using the 'Big' frames?

Are you using the 'Big' frames?

I am

So far we haven't received any reports regarding this issue, except from you. I'm also pretty sure this is not being caused by SUI.

Era Classic and Cata are running on pretty much the same code base, and I haven't seen this issue at all.

Please make sure to disable any Unitframe AddOn that you have as it might interfere with SUI.