That’s weird, I have perfect running detection on an old(er) (Apple) watch. Detection does start late (but is retroactive, so it’s not an issue).

And that's by design, because it waits for a larger window of data to gain confidence. Notice how you ~never get false positives on it starting an exercise session.

I see comments like GP's often enough that I think Apple just does a bad job explaining how/why it works.