Does it do it with clutch pulled in , could be a loose caliper bolt , loose drive belt , loose primary chain Keep looking

check gearbox & pri oil 1st ?

I have just discovered what I think may be what you are describing on my Roady . I have reasoned out that when I roll back after stopping & use rear brake it must somehow grab the back edge of the pad & then when braking in forward motion it  snaps back .its just a theory as I`m yet to examine