UPDATE: Sharing a bunch of learned info in case it's useful to anyone else.
After extensive additional research, Google pointed me to a forum post here that I hadn't previously found that appeared to describe *somewhat* similar issues.
A response therein pointed to this FordTechMakuloco video which describes known issues with the electronics within the 6R80 transmissions that seem to track, and how to fix them:
In this video we show you how to make the 2011-2017 Ford 6R80 as reliable as possible!These are the Tools and Parts I Recommend:Updated Molded Lead Frame-htt...
www.youtube.com
In between then and now, I had new, different symptoms emerge that appear to confirm it:
1) Transmission failed to shift out of 1st gear upon gentle acceleration from a stop (speedometer was working). Pulled over, turned off the ignition and restarted to resolve.
2) A single somewhat-hard upshift a few miles later (speedo still working).
Days later (today), things got worse:
1) Transmission shifted up and down erratically a few times at slow speed, then went into top gear as before. Speedometer was at zero. Restart didn't fix, at least not for long.
2) Between then and when I found a place to pull over again, I got a "_ _ miles to empty" warning on the center display (tank was half full).
3) Restart resolved it and I got home without any additional problems.
After my original post, I did check for any related OBD codes, and found none. Checked again after today's experiences and found two, both related to the Output Shaft Speed Sensor Circuit -- P0720 and P0722 -- the first of which was called out in the forum post I found.
I also found a second forum post here that described symptoms more similar to mine (including the out-of-fuel warning) that the OP resolved himself by following Makuloco's video.
Bottom line: The OSS sensor is part of a one-piece electronics component bolted atop the R680 transmission's valve body called a "Molded Lead Frame." It also houses the transmission range sensor, which could explain the shifting issues seen today that didn't coincide with a zeroed speedo.
Replacement part supposedly addresses both sensor issues, and includes an added rubber seal that keeps metal particles from creating conductivity between the transmissions solenoid connectors the Molded Lead Frame mates with (another known issue).
Additional notes:
I did check for TSBs related to the issue; found none pointing to my model. Odd given that Makuloco said Ford updated PCM software to keep OSS sensor issues from resulting in a dangerous rear-wheel-locking sudden downshift at high speed.
One comment in the aforementioned forum posts reported similar issues caused by trans fluid getting past the bulkhead seal for the electrical connector. I will check that, but the only way to replace the seal is to drop the pan (also part of Makuloco's fix). May as well replace the Molded Lead Frame while there.
The process is the same as doing a fluid & filter change, with added steps to drop the valve body to replace the Molded Lead Frame along with a couple of seals.
Makuloco recommends doing it proactively if you have an affected transmission (at your next fluid & filter change perhaps?), as encountering these problems seems inevitable.
The two related earlier threads can be found by searching for the YouTube video link listed above.
