Not a discrepancy as the cloaked ship In TOS S1 Balance of Terror could be detected via motion sensors if it was in fact moving.There are many cloaking discrepancies in Trek, like cloaks being detectable by motion sensors in "Balance of Terror" but not in "The Enterprise Incident,"...
That's why they had the dialogue in TOS S3 The Enterprise incident that the Romulans had developed a brand new cloak that rendered a ship completely undetectable whether it was moving or not.
In fact back in the day, some fans would claim this was a continuity error when in fact it was not because the Federation had seen a Romulan cloak in Balance of Terror, but these fans had forgotten that it was just a visual cloak and a moving ship could still be detected.