Is this some safety feture or what? Why did the RB750UP shut down the POE output?
when voltage drops 500mA limit stays the same, as result power output drops too, if at 24V * 500mA = 12W, then at 20V output is only 10W etc. if that is over the budget then power is turned off.
Also, when there is load, there is small additional power drop, so as result, when one or more devices go off, voltage rises a bit thus enabling it to work further.
Also, if you are powering devices with wireless, then lower data rates use more power then higher ones, so if link goes bad and AP/client has to transmit frames over wireless link at lower data rates, it uses more power.