As shown in the picture below, player "_Tarcadia" is controlling the boat, while player "Tarcadia" is seated on the back seat. The log shows the event calling, that PlayerMoveEvent fired for "_Tarcadia" but not fired for "Tarcadia".

This ignorant of PlayerMoveEvent covers players' riding a vehicle in secondary seat, and players' riding on an entity (pig for example) which is riding on an entity's secondary seat, and players' riding on a minecart.

This is similar with the problem introduced in issue: https://hub.spigotmc.org/jira/browse/SPIGOT-4568

But this issue covers a much wider range.