Having the quarter screen problem on both of my 2018 Ultra's hooked up to a Vizio E70-F3. Predominantly see it on Youtube TV and Youtube. Also, both Ultra's briefly show an HDMI error screen when some commercials come up on Youtube TV, The message clears relatively quickly, but extremely annoying nonetheless. For reference, neither a nvidia Shield not the Firestick 4k have this issue. Also, numerous high speed (up to newer 8k) cables have been used with no resolution to either issue.
I realize there is a thread pertaining to the Express+ with the same quarter screen issue. Let me know if you'd like to merge.