We have a 2017 MP and a few years ago had the roof replaced under warranty. This was originally reported in May 2021 when the van was just shy of 4 years old (and 8000 miles). I feel it is likely this is osmosis given the roof is GRP, but osmosis normally takes 25+ years to appear in boats so this is very early. Our roof had also cracked down the near side which I suspect was caused by raising the roof with an (empty) Thule roof box and the position of the roof bars relative to the raising levers. Sadly these roof panels are probably too thin and would benefit from beefing up particularly on the lower edge of the sides (which is the area likely to suffer the greatest tensile stress and in my case would have been the point at which the crack/split first appeared before propagating upward). Unfortunately the warranty on the new roof (as a spare) is only 12 months so if we get more blisters it's just tough. If it is osmosis then structurally this is unlikely to be a concern, it's just annoying and as with boats becomes a negotiating tool.