I am an experienced engineer, but a novice timber framer. So, take these comments for what they're worth.
Looks like you've still got about 3/4" of bearing between that beam and the post. That's probably sufficient, but you might want to check your loads.
It's also fortunate that the check is perpendicular to the pegs, so it doesn't compromise the tension-perp-to-grain strength.
My guess: you don't have a structural problem here, only an aesthetic one.
I don't think you're getting any wedging action from the rounded edge tennons.
It looks to me like you're getting splitting and shrinking due to drying. I know you said they are RF kilned. Did you put a moisture meter on them? If so, did you check internal MC?
I might be barking up the wrong tree, but isn't RF drying pretty tricky to get right? Maybe the surface of the timbers were dry, but internal MC was still high.
That post sure looks like a "cut green, dried in place" timber to me.
