i now know. Apparently the upstream sensor is not clearly visible from the underside.

The mechanic changed the downstream sensor, which for some reason turned OFF the Check engine light and the codes. He didn't see the upstream one, or he didn't know to change it.

When he ALSO changed the upstream O2 sensor, the problem appears to have gone away. Test drive yesterday went fine. Much longer one today.

 

I bet the bozo disconnected the battery when he replaced the sensors, doing that clears the OBD2 and makes the testing start from 0. Testing can take 10 cold starts to complete.
Any mechanic should know that ALL cars made this century have at least 2 O2 sensors. Find a shop that knows what they are doing.

 

I'mma gonna let the service advisor (supervisor) take care of the mechanic. He went out with it after I drove it around enough to replicate the issue. He chewed out the mechanic for a) not doing a test drive when he said he did, and 2) Not changing out the second O2 sensor. The service advisor agreed with me that it should have been changed with the first one, and the return trip would not have been necessary. That was all he had done when he brought it back, and it's fine now.

Standing behind their work is why I keep going back to them. If it weren't for that, I'd have serious problems.

 

Ok.