Logging onto a forge server without forge 1.19

When transferring a server from 1.18.2 forge to 1.19 forge. I noticed something extremely strange happen to the blocks, mobs, sounds and item drops when logging onto the server without forge 1.19.

What I expected to happen was...:
The server allowed people to join without forge on 1.18.2, so what I expected was a normal Minecraft world being loaded when joining from 1.18.2 to 1.19.

What actually happened was...:
I joined the world, and all the blocks were just, like the debug stick had been taken to them. Waterlogged stairs were acacia planks, wheat was redstone dust, endermen were literally walking end crystals that make ender dragon sounds as just an example.

Steps to Reproduce:
1. Load an Aternos server with the latest forge version for 1.18.2, load the mod Xaero's Minimap as well.
2. Log on to the server and play around a bit, everything should be normal.
3. Log off the server, and swap everything to 1.19, meaning, forge to 1.19, and the minimap mod to 1.19.
4. Join the server without loading forge and the mod. It should be an entirely different world (You'd start with the trees being different wood types)
5. That's it!