Mine was parked outside ,noticed first blisters about Feb 2019, had a new roof fitted within 3 months, but the second roof had 2 large lumps on the roof ,the workshop at MB tried to tell me it was to be expected as it is fibreglass, needless to say I rejected it .took another 6 months to get sorted 2nd new roof ,looks fine at momemt. guess I need to wait for some really cold weather,
However I am certain the 2 lumps on the 2nd roof became more prominent from the sun, I think there must have been air expansion as caused a small bump in roof inner lining as well.
I asked why it had not been recalled and was told it was not an issue to cause an accident so would not fit recall criteria!!!!