RootVehicle tag doesn't exist when 2 players are riding a boat

To detect if a player riding a boat, I use this command:

/data get entity @s RootVehicle

This works when one player is in a boat, but when a second player enters, this tag doesn't exist anymore. I get the following error message:

Can't get RootVehicle: tag doesn't exist

Only players as second rider do not work, other entities (like zombies, pigs, sheep) work fine.