A previous operator had a number of aircraft that would exhibit this behaviour. Traditionally in open climb reducing speed would produce a pitch response to meet the target but one software fit would trigger the auto thrust to reduce thrust instead. I remember the first time I saw it there were a few expletives and why is it doing that. The software changed a few months later back to what I had been used to.
Open climb should be a speed based pitch mode with auto thrust commanding climb thrust, not the autothrust commanding speed