Moving a server to be behind BungeeCord, how do I get players to keep their inventories and spawns?

Hi,

I was previously running a small FTB Builders Paradise server for my family. I'm now trying to put it behind BungeeCord so I can run multiple Minecraft servers and people can connect via subdomains instead of ports.

I've got this working and I can join the server, but I've lost my inventory, my spawn point, and any other player-specific data. Is there an easy way to migrate my existing player data to match whatever IDs BungeeCord uses? Or is this meant to happen by default and...

Moving a server to be behind BungeeCord, how do I get players to keep their inventories and spawns?