Emrakul and Scrying
See attached screenshot.
While controlling my opponent's turn using Emrakul, the Promised End, I activated their Castle Vantress in their main phase, and was presented with what's shown in the screenshot. I was unable to view their top card, or move it to the bottom. I could move the Hallowed Fountain either up or down, but I could not interact with the face down card.
After I had completed scrying, the mystery card became visible to me as it should have been.
Perhaps notably, the face down card was one that my opponent had previously scryed, using the Castle on my turn after I cast Emrakul and leaving both cards on top
While I'm here - please please consider moving away from this horrendous bug report system of "posting ideas" and voting. I am a very experienced bug reporter across multiple games, and have run into quite a few minor issues on Arena, but I simply cannot be bothered to navigate this godawful system to report most of them. This system is clearly not built for bug reporting.