Di2: RD won't stay on big cog
 

This happened completely out of the blue tonight. Swapped the chain from one bike to the other, didn't adjust anything, didn't bump anything, nothing at all, and all of sudden, whether in the big or small ring up front, when I shift into the big cog, the RD (R8050) clicks for about 5-6 seconds and then shifts a full gear over and falls onto the next smallest cog. Not like a small chain-line adjustment, but a full shift over. I've tried resetting to default and it's still doing it. Never seen it do this, and definitely bewildered after adjusting nothing to cause it.

Swapped the chain from another bike to this bike? Same length chain?

Was the chain you took off very worn to more than 1/16" in 12 inches? If so, maybe you wore out the cog, but being the big rear cog, are you in it that often?

Didn't swap wheels did you?

Your bikes not upside down is it? If so, then turn it over and tell us what it does when you ride it.