Radiants gain access to their Plate upon swearing their 4th ideal, not their 5th. So it would make complete sense for us to have seen evidence of Jasnah summoning/dismissing it if she had just sworn her fourth ideal, as I believe that she had. But prior to that moment there is literally zero evidence that she ever had access to her plate, including in at least one situation in which it absolutely could’ve been useful, namely when they were attacked by the Ghostbloods on the ship.
So I’m not quite sure why you’re putting this forth as evidence against my theory, when the reality is that if anything it’s completely expected under it.