Before I get started, my background is ex-FAA aircraft Dispatcher and ramp rat, now a physicist employed as a GPS Security Technologist working for a major GPS simulation/test company.
I've been monitoring the reports from the Black Sea - there were reports from Moscow and St Petersburg in late 2016 of possible GPS spoofing taking place - in Moscow, users near the Kremlin reported that their GPS receivers were indicating that they were at Vnukovo airport, a significant error.
Here's one of the reports: -
Getting lost near the Kremlin? Russia could be 'GPS spoofing' - Dec. 2, 2016
But we do have to treat all of these reports with a little bit of caution - we do know from work that was done in the UK that GPS jamming can cause GPS receivers to output misleading position and timing information - this happens when the interference level is significant enough to affect the operation of the GPS receiver but not enough to saturate the RF front end and stop it working completely. The project in the UK showed this graphically - with some unexpected results...
https://rntfnd.org/wp-content/upload...-GNSS-2013.pdf
So there's a lot to consider here before we could say what is going on with much certainty. Some of the reports (the 22 June report to USCG) is interesting as it does suggest something a bit more than simple jamming might be going on.