Well, in this breakdown of the output shaft

there is a shim (part #19) that is available in 6 thicknesses of 1.0mm to 1.45mm to space the clutch basket position on the output shaft.
This shim spacing on the shaft will also have an impact on where the seal face contacts the clutch basket hub.
When I did my 1st clutch job, I had this shim come out with the clutch basket. I almost lost it....... It could be possible that the p.o. had this happen and either lost the shim or found it later and didn't know where it went.
Not saying that this shim is gone on your bike, just a possible explanation why it won't keep seals intact.
I do not know how to determine what thickness shim is required if it is gone.............
Another potential possibility would be the bearing(#11 in the image) being worn and allowing improper positioning of the clutch basket in the seal. IF it is the bearing I would also think that it would cause vibration and excess wear on the tranmission input shaft bearing and/or trans input shaft seal leaking( you did say that the p.o. did replace the trans input seal).