You’re jumping the gun.
They do show a problem with Stamets taking the place of the Tartigrade. He seems to split in two after foreplay, er, tooth brushing, with his lover the doctor.
There’s a bit of dialogue in the next episode where Stamets assures the captain that everything is fine, but we see him acting a bit differently than episode 3. “Groovy???”
Previews for this weekend’s episode highlight that the ship gets caught in a 30-minute time loop or lag, which, we can guess from Stamets’ post-foreplay mirror lag, has to do with their current spore drive solution.
This Trek is serialized, unfolding over time. Give ’em a chance.