Why it is not technically a defect, and is still a problem
A lease that obliges each tenant to insure their own demised part is doing something the law allows. The Lexsure wording below is careful about this: it is not a defect in the lease, it is an arrangement with practical difficulties.
The difficulty is structural rather than legal. You can insure impeccably and still be unable to rebuild, because your flat cannot be reinstated in isolation from the building it is part of. Your protection depends on strangers.