To repro this bug:

1. On a private Windows 10 Minecraft server, have the Operator/Admin player promote another player to Operator permissions level in the Escape Key/start menu
2. Have the new Operator logout of the server
3. Have the new Operator rejoin the server

Result:

New Operator player is reverted back to Member permissions level when rejoining the server

Expected:

New Operator player retains Operator permissions level each time they join the server