Lots of possibilities:-
alignment, o/ring compound, thrusting, oil grade.
I would look seriously at alignment, I prefer Lazer alignment, zip /zero mis-alignment.
Any thrusting will kill o/rings. Motor bearings should C2 or C3 rated.
The shaft seal while running is pressure feed with oil so all systems are fine,stopped the seal box drains and any suspect o/rings will leak, and those stupid wedge rings.
Start with alignment checks