Have never used the HP canvas, but canvases with heavy weave texture crack more readily that those with finer textures. That HP canvas seems to have a very coarse texture.
With canvases like that your best option is to make the smoothest possible folds and bends. Don't pull too hard when you are working on the corners, and don't pinch the folds.
And of course stretch the canvas soon after coating. In my dry climate the best time to stretch is just a few hours after spraying while the coatings are still soft and flexible.
Just a wild guess here...you said you sprayed it with matte varnish. Are you using Glamour II Matte without dilution? You should always use a mixture of gloss and matte. The matte varnishes have very fine powder suspended in the solution and are not as flexible as the pure glossy varnishes.
