Hi, I would also be grateful to understand how this worked out as my 2016 A7 Black Edition has same issue, initially thought it was condensation in the unit but it’s got worse over last couple of months, looks like crumpled tinfoil and projected image on windscreen is completely obscured.
Many Thanks
Have a look through this thread (Register to see the images):
HUD Display looks scratched ie like foil
in Audi A7 Club
Posted
Have a look through this thread (Register to see the images):
https://www.vwaudiforum.co.uk/forum/showthread.php/184679-Replacing-Head-Up-Display
It explains the issue and a place that recoats the mirror.
It happened to my A7 and is a VERY fiddly job without removing the windscreen!