Mokva air defence radar only looks one way.
They made it look away from the coast using a drone, and then launched missiles when it turned away from the coast — this is how I imagine it happened.
But I have a hard time picturing the Moskva having trouble destroying a drone with the performance of a WWI-era plane.
It also had two radars, MR-710 and MR-800, but I'm unable to find if they have differences in terms of target tracking and acquisition.
Given the Russian army's current level of performance and the Russian navy's...entire history, I don't have any trouble believing the Moskva was beguiled by a drone.
They have a small RCS, but they're slow as a dog, and signals at sea are significantly easier to interpret given that there's infinitely less spurious reflection and radio sources. Maybe the bad weather played a bigger role in that than I've seen accounted for. But even with detection at close range, any modern heat-seeking SAM should have absolutely no problem taking it down before it drops anything funny on target.
Additionally even if drones were detected the Moskva may not have been equipped with heat seeking munitions. Even if they had them, a drone at stand-off distance can easily be out of range of their targeting. The drones also only need to occupy the ship's attention long enough to distract it from the incoming anti-ship missiles. Even if the drones are shot down it's a small price to pay to disable or sink the Russia's biggest naval asset in the Black Sea.
Finding things even in calm seas is hard. Aiming at those things is harder. Hitting those things with a fast moving thing is harder still. Poor equipment increases those difficulties as does poor crew capability. All of those combined makes for single use submarines.
That seems like a pretty big flaw, given how easy it could be to, y'know, attack from more than one direction at a time.