I might RTFM. Considering how much a heated rear window eats your MPG this could be a factor.
Why would the adjusting mirrors position turn them off anyway?
It just does.
Other day, frosty moring, thought the feckers had broken. Done 20 miles and still could'nt see out of them (minus 5 outside) anyway looked down at switch and was to the left from the night befores parking.
Tunred back to centre and cleared within mins