When you installed the drive did you glue the large rubber ring to the front of the bell housing and when the drive was installed if it was not supported and you “wrestled” it on there is a good chance the large ring got caught and unseated the ring.
I was the outdrive gasket . the silicone grease is the gasket outdrive ..
inspection of the alum corrosion !!!!!!!!!!! no glue .
then the corrosion of the alum of the gasket area ... then the marine-tex epoxy ,,, wire brush and the alum cleaned it , and then the marine-tex ...
I was my outdrive 40 yrs , no corrosion , no water inside the drive shaft bellows ..
I had a similar experience my my old boat which had a Alpha gen one. Had a bad gimbal bearing, pulled drive (first time for me) and found small amount of water in bellows. Could not find a leak or tear in bellows. Put drive back in of course was too green to glue in big ring gasket. Put drive back on after doing gimbal, next time I pulled the drive there was no water in there- had no issue with u joints ... Now with more experience thinking maybe the shift shaft seal but who knows.
Big issue is if you get a lot of water in there and never take the drive off, Input yoke will rust and mess up seal. On the older drives with out the lube bottle if you don't watch your level, the gear oil leaks into bellows until upper runs dry...