Above (won't let me edit) I typed that a plug had gone bad without setting a code. What I meant was that a coil had gone bad. It did not flag the code for that cylinder until after it was replaced. Cleared the code, then took the bad coil (numbered as I removed) and put it into the other bank. Sure enough the misfire returned, and did not set a code (did not run it that way long).

The unburned fuel from a misfiring coil ends up in the catalyst and combusts there, potentially overheating it or plugging with carbon.

 

Update - As I mentioned earlier, after replacing the MAFS, the engine smoothed out quite a bit but I still had a roughness especially on hard acceleration. I couldn't find any codes and rather than experimenting, I paid the Jag dealer $130 to run diagnostics. Turns out I had a broken engine mount. I couldn't believe it. I would think that I would have know that. But it didn't jump or thunk like what I would expect. I just felt an uneven vibration that I attributed to engine performance. I replaced the mount (fun) and it feels great.

Just FYI - I still occasionally get a faulty MAFS code. The tech said it's not unusual at our altitude (about 5,000 ft). Just erase the codes and don't worry about it unless it becomes frequent or won't reset.

Thanks again to everyone who contributed. I didn't want to leave it open-ended in case someone else experiences something similar.