The recommended cg is 1/2" back from le, measured at the wingtip. I fly mine 1.5" back, very little change (flying inverted) from 1.2" cg setting. I suspect it has to do with wing incidence (engine thrust is at 0,0 and flies fine upright), maybe even that built-in washout at the wingtip.

Its a fun little sport plane. It is heavy though, and tough as nails. I have never tip stalled this plane (unless it was on purpose), not sure why you guys are having such issues.