I have had to speak with Tesla charging support a couple of times over last 8 years. They have been very knowledgeable and helpful. If the issue is with one of your wall connectors they can ship you a replacement.
Thanks for the detailed troubleshooting. I will be returning from vacation soon. I also opened a service request with Rivian. Did anyone else do that? The person I spoke with said he would put in a feature request.
To eliminate it as a Rivian issue rather than a J1772 or adapter issue, try another J1772 vehicle or a different adapter. Would also be curious if 2 non-Teslas work or not. It is possible that the pairing software is not able to reduce the charge rate on the J1772 connector once started and will not start the second vehicle. Does the Rivian also fail to start if the Tesla is started first?
I also put in a Rivian service request. The initial tier that called me to discuss was not knowledable about EV charging and kept telling me that if my Rivian works on other chargers then it's not a Rivian problem. They also offered to put in a "feature request" for future vehicles. I don't think that's a useful resolution for an issue that needs to be fixed for all current Rivians (I suspect). The agent said they would have a higher tier with more charging knowledge call me back. I unfortunately missed their call... still waiting for a second call back!
What about when no Tesla or second car is plugged and Rivian still stops charging after about 20-25 min? Like I’ve said in prior posts, I just lower the Rivian to like 32-38 amps and just deal with the slower charges (in the summer)
In my limited testing I have not seen that issue. Solo, the Rivian stayed at 48amps for as long as needed. I will continue to test.
Granted I’m in AZ and my garage hovers about 100 degrees in the summer
). 
. Of note - I'm also in the mountains of CO and the temperature typically maxes out (outdoors) around 80 degrees. I think i've seen 90 degrees in the garage (it faces west) once, fairly briefly.