Well.... there is this one boulder.
I doubt ejecta would be spherical enough to roll around on the surface — it would be jagged and wouldn’t roll. Most ejecta is more of a splash of a semi-liquified rock rather than large roundish boulders.
With the weak gravitational attraction, a boulder would have to be very large to create enough force scar the surface of Phobos, but the small tracks indicate small boulders.
Could the boulders roll into the craters and have enough momentum to carry them up and over the opposite uphill side? I would think that much momentum would launch them off the surface.
Lastly, where are the boulders? There are none to be seen.
Also not much gravity on Phobos, I would image all the ejecta blew into space.
Well.... there is this one boulder.