Workaround
You can download the server jar here.
The Bug:
The snapshot 21w20a article links the 21w19a server.jar instead of 21w20a.
Steps to Reproduce:
Head over to the 21w20a article. https://www.minecraft.net/en-us/article/minecraft-snapshot-21w20a
Download the Minecraft Server jar.
Launch the server.
→ ❌ Notice how you are required to play in 21w19a instead of 21w20a in order to join the server.
Workaround: You can download the 21w20a server jar from https://launcher.mojang.com/v1/objects/054b2065dd63c3e4227879046beae7acaeb7e8d3/server.jar until this is fixed.