It could be any number of things, including sever chainring wear, but the fact that it only happens under load may be a clue. It could be that flex under load is moving the top of the chainring outward with respect to the FD cage, causing a ghost shift to nowhere.

Try this experiment. Shift to low. Hold both brakes (or pit the front wheel against a wall) and put the right pedal at the forward horizontal position. stand on the pedal and load it while watching the chain in the FD cage. Does it move outward enough to touch the cage? If so, that's why it's shifting.

It might help to adjust the inner limit outward a but, but the real solution is finding out why it flexes so much. Some may be normal frame and crank flex, and can't be fixed, but there may also be play in the BB which needs to be eliminated.