Also, I checked the call sheet again, and DeForest Kelly is also listed as H for "Hold," while the other four actors are listed as W for "Worked."
I did some research and jogged my memory, and I've remembered what "hold" means. It means that the production isn't using the actor on that particular day of shooting, but isn't done with the actor yet, so they're still being "held" by the production. So the "hold" notation for Kelley and Doohan meant that they weren't on the set that day, but they still had scenes to be shot on a later day. Maybe they were supposed to stay available in case something delayed shooting on the intended set and they had to substitute shooting a different scene that would've used those actors.
Maybe they were just being held to complete some ADR work to finish the season?