[RexNfx400]

If the transmission is checked and verified to be in good operating order(Extensive road test). I would see no reason to not pull it out and see whats leaking. Once its out, you have to pull the torque converter off the trans. Then pull seal off transmission and inspect bushing in the converter and the support the bushing rides on, on the transmission. Some different possibilties happen next:

The converter hub bushing is worn excessive. Which could damage The stator support (The transmission has to come apart to change support)

The converter hub bushing is worn, but support ok, so replace bushing and seal.

Just the seal is loose on converter hub. Replace seal and ship.

Thats all the normal stuff... Unnormal stuff is:

Torque converter welds fracture and leaks ATF.

Leaks through channel plate bolts into bell housing.

Case fracture.

You only go after these leaks after proper operation(shift timing, shift feel, TCC, egagements, fluid condition, normal temperature) is verified, perfect.