You have checked for low fluid level, loose or mis adjusted gearshift linkage/cable,so we can count this problem out.
Your technician said all looks okay so we can count out burnt rear clutch,
So that leave us with the following possibility's like a valve body malfunction,
or broken overrunning clutch the biggest possibility, or a worn/damaged input shaft
seal rings or a broken front planetary.
This is all that this could be for this concern.
Your tech will have to pull the transmission and recheck these possible causes .
Kind regards Greg