I wouldn't discount there being a design flaw, but it ain't necessarily down to the use of scissor hinges.Design flaw! Scissor hinges lethal to canvas
My roof has scissor hinges and I've never had the slightest problem. As @The Flying Scotsman alludes to, a well-designed canvas will have elastic straps that provide inward tension, thereby keeping the canvas away from the hinges and preventing it from billowing outwards when the roof is dropped.
A bungee strap of suitable length, stretched around the girth of the canvas is a simple alternative for those roofs that don't have an inbuilt tensioning system.