I found this thread this evening and it has given me some hope! Starting in 2019, I had the same issue as Grant, paid for a whole new mirror unit only for a second failure. Further testing done and gave up at the point where Skoda technical said same for me, replace (or at least take out and troubleshoot) the wiring loom all at my own expense. Garage said a "complete wiring continuity testing between the mirror control unit and the central electrics control unit."
Am I heading for another burst bubble here, I'd really love for the high beam assist to work more than the one time it did and then failed 😞
The garage gave great detail at the time if it's any use but I am eternally grateful to any knowledge from here that can be shared. Thanks!