How to keep the bias current constant in class AB output stage?
The top transistor (BD139) supplies positive current to the load as well as bias current. It doesn't draw current FROM the load, only the bottom transistor does that - which is why it's called a push-pull amplifier.

You're only measuring the "push" half of the AC output waveform (in addition to the bias current) across that resistor, which, on a multimeter will look like a DC current.

So the increase you're seeing is NOT bias current but output current. 

You can't eliminate it, other than by removing the load impedance or the AC input.

(Also, @Bimpelrekkie is right, there's an emitter resistor missing on the first stage)

EDIT : if you wish to measure the bias current separately from the output current, you'll have to measure the voltage across the 0.33R that is NOT currently supplying output - i.e. the lower one during positive half cycles, or the upper one during negative half cycles.

This pretty much requires a differential probe (or the difference between two channels) on an oscilloscope.