This is a known bug, it has to do with flak 41 damage model, and when ai tries to repair it that happens, one way to fix assuming you playing coop and some players can still see map if flak 41 is destroyed then u will see everything again. I have already reported this bug to DMS here:
http://digitalmindsoft.eu/forums/sho...l=1#post233886
DMS is currently testing balance patch on steam, hopefully they include fix for this bug as it is bad when it happens.