I can't say that I have ever had a wing fold up like your describing here ,but I agree theres something wrong with the structure for that to happen .while the covering does help with wing strength ,(example) a sheeted wing should fly weather its covered or not and not fail.
I have used coverite with great results and its a heck of allot stronger than the plastic film coverings,but you pay for that strength with extra weight .Sig makes a fabric covering thats both lite and easy to put on ,but you need to prime and paint it afterwards .It holds up purdy good I just finished stripping a plane that was done in it 25 yrs ago .