“Flying upward” as you describe when hand throw test gliding is called stalling. Hand test gliding accurately is actually more difficult than it sounds. This is the step I always teach first to my students and few can learn it in one session (I demonstrate and they attempt several times usually with the same result you describe).
The arm and hand motion for a test glide should be very linear, like throwing a dart that is very light, and aiming at a spot on the floor about 15 o 20 feet away. Any bigger or “arcing” arm and hand motions usually result in a slightly “nose up” release, which results in a stall.
I just tried your suggestions about hand throw test gliding it. Its not stalling much anymore, and it honestly flies pretty well. However, my concern is that with the extra force used in a rubber launch, the plane will still stall and loop upwards.