Bike fit / correct riding position

I get this. I feel that cycling keeps the hamstrings stretched more than running. Which continually stretches and relaxs them to a higher degree.

This means going for a run after a lot of cycling actually feel Nice, as it seems to loosen everything up. While going for a ride after a lot of running does the opposite.
I think you might be on the right track - I also definitely find running after cycling a really nice experience. I think I'll check out a physio local to me who has had a lot of experience in cycling and triathlon as I'm sure he must have come across this before.

I'd check sleeveless sites.