I mean, this is pretty much why Russia even agreed in the first place (it was definitely NOT their newlyfound humanity, Russians are free of such burdens). UK was in actual talks to send their fleet to protect these ships and most likely under the table told Russia that their compliance is not necessary. USA already told Russia that if they see a nuke they will perform a magic trick that makes Black Sea fleet disappear which also implies using navy.

So now that Russia is officially withdrawing from this deal they are effectively testing West. We have the means to enforce it, now can we actually act on it? If NATO vessels actually arrive in the region you can bet that Russia will instantly go "oh, deal is back on, we have got what we wanted from our negotiations". Since let's be fair - British fishing boats have tied with entire might of the Russian Baltic fleet before and it's safe to assume Russia has not improved much since.