One of the PW-1 devices I have (purchased as: "broke - for parts") is totally bonkers at less that 80% battery charge.
It is only "normal" full charged - I have learned: Leave it plugged in to a USB hub (external power) - then it is always happy.
It almost seems like (something's) operating voltage tolerance and the voltage output tolerance of one of the voltage regulators (there are quite a few of them) are at opposite ends of there specifications.
The sort of thing that can happen when you toss a million "normal" parts in this bin and match them at random from another million "normal" parts in that other bin.
They just don't play nicely together, although both are 'in spec'.