Pretty sure if the crankshaft to bearing mating surface is leaking you would need to test with the bushing in place over the o ring or else it will have no force applied to the o ring to form a seal. Basically have to test with the clutch installed, perhaps not or at least not fully torqued. Would like to think the outer seal would hold it in place, maybe not. Seems a weird situation.
A well used saw will need a new bushing, also if the bushing has been heat blued it should be replaced as it may no longer be round.
In the last couple months have done 6 365/372 crank and or bearing replacements, have not had issues such as this. It would be frustrating. Hope you get it sorted, good luck.
Have another sitting on the bench now, probably just jinxed it, gonna be a leaker.