I also teach this stuff. The only TR malfunction that requires "land immediately" is loss of drive to the TR. Loss of drive to the TR is the most obvious malfunction to diagnose in the whole book of malfunctions. We know they didn't have a loss of drive to the TR, because if they did we'd know about it because that is serious need to know stuff for the whole industry. Of the other TR malfunctions, and there are 3 of them, the technique to diagnose and land is basically the same for all of them, and none require "land immediately". In teaching this stuff I find many trainees don't even realise they have a TR malfunction until they are getting setup for the landing. I think reasonable to assume we are not looking at a TR malfunction of any sort here.
So something has happened to get their attention. Hard-over in the yaw channel of one of the AP will get your attention for sure. Again, this is very easy to diagnose. Just look at the Yaw channel series actuator position indicator. It will be all the way to the left or right. Hunting in that channel will also be evident by the twitching response of the aircraft and the position indicators.
Anyway. We can speculate until the cows come home. Nothing makes sense yet. I hope it will, eventually. I would like to learn something from this so I can teach it to my trainees.