Article 92 also lays down that: “A ship may not change its flag during a voyage or while in a port of call, save in the case of a real transfer of ownership or change of registry.” This is often misunderstood and assumed to mean that a change of flag in mid-voyage – such as appears to have occurred in this case – is not permitted at all. But a closer reading reveals that this is not the case. What it prevents is a change of flag without a corresponding change of registration.
But that is not the position here. Assuming there was a real registration to Russia, that is what counts.