The bug

When a player uses a 1.16.4 client to connect to a 1.16.5 server, that player's DataVersion value in their NBT is the server's data version, not the client's. Since a player's data version is tied to the player, it is logical for it to have the data version of the connecting client, not the world.

Steps to reproduce

1. Fire up a 1.16.5 server that you had lying around
2. Connect with a real 1.16.4 client
3. Run data get entity @p DataVersion from the server console

Expected result

<player> has the following entity data: 2584

Actual result

<player> has the following entity data: 2586

Other info

Data versions: