Definitely take it back to the shop that had the transmission open for service immediately prior to the onset of symptoms.
I doubt it is the pump, that would affect all gears not just Reverse.
The reverse clutch could be not engaging.
when in R, the R clutches and low-1st clutches are engaged.
That is the only time the R clutches are engaged, the low-1st clutches also get engaged if in Manual-1st gear.
So, if the trans seems OK with the gearshift in manual 1st, I would think the reverse clutch is not engaging for some reason.