It's related to the close coupling and the high camber of the airfoil. Basically the strongly cambered wing has a strong pitching moment that is related to flying speed. At lower airspeeds the stabilizer has enough authourity to control this strong nose down pitching due to the airfoil's camber. As the speed builds the pitching moment force builds faster than the stabilizer's ability to control it and the result is the tuck. Part of this mismatched rise in pitch damping may be due to how the downwash off the wing impacts the stabilizer as the speed changes as well as the general lack of pitch authourity of the stabilizer due to the ultra short moment arm compared to wing chord.

I saw this same issue with a friend's Slo-Poke, another very short coupled "cute" model. It's the price you pay for a design that is so extreme. If the various 'Bee's and 'Pokes used an airfoil with a lower pitching moment or had longer tails then this would not be an issue.

You can try moving the CG ahead a bit but then it'll tend to have other "issues" like wanting to nose up strongly with the addition of power. Adding more downthrust can control that. But my own feelings are that the tuck will still occur but just at a slightly higher airspeed. If a 'Bee or 'Poke was made with a pitching neutral or positive pitch stability airfoil then this tuck issue would not occur since it would sort of be like a plank style flying wing with an added pitch control. Basically with that sort of setup the wing would be stable without the stab and elevator and that surface would merely be there for pitch control.