Original bug report by @Moresteck.

While launching versions between a1.2.0 and a1.2.6, the player name is always set to:
"Player" + System.currentTimeMillis() % 1000L
instead of the actual player's name.

This can be simply solved by adding a tweakClass minecraft argument from other Alpha jsons to these Alpha 1.2.x json files:
"minecraftArguments": "${auth_player_name} ${auth_session} --gameDir ${game_directory} --assetsDir ${game_assets} --tweakClass net.minecraft.launchwrapper.AlphaVanillaTweaker"

It's simple to fix, and if fixed, it will stop making people search for launcher alternatives just to join Alpha 1.2.x servers.

Here are the reproduction steps:

First you need to step up the environnement:

1. Download a copy of the Minecraft Server from the official wiki: https://minecraft.fandom.com/wiki/Java_Edition_Alpha_server_0.2.8
2. Launch it once, then type stop to shut it down.
3. Edit server.properties to turn online-mode=true into online-mode=false
4. Start the server again.

The environnement is now set up. Now we do the test:

1. Inside the Minecraft Launcher, choose the Alpha 1.2.6
2. Launch it
3. Go into multiplayer and type localhost.
4. Notice that your username has been totally forgotten by the server, and instead you are called something like PlayerVVV where VVV is a 3-digit number.

You can find the media with.mp4which shows the bug fixed with the added parameter, and without.mp4which shows the current situation (buggy)

Also sorry for the media quality.

Also, note that the change has a side effect, at least for alpha 1.2.6: when closing it, the launcher will think that it crashed. (It can easily be fixed by patching the LegacyLauncher component:

Changing:

System.exit(1);

to

System.exit(0);

at line 91 in file src/main/java/net/minecraft/launchwrapper/injector/AlphaVanillaTweakInjector.java

)