ORIGINAL: destructiveTester
....I had a trainer with the tip shape you mentioned with the bottom flat and angled up. Terrible tip stall into a spin at the drop of a hat.
I suspect your tip stall was due to other factors. I've used angled up tips with great success on two of my designs and seen this method on many others over the years. The classic use of this being a flat bottom airfoil where the tip outline is a stretched version of the airfoil with the bottom beveled up to meet it.