The Bug:

Using a custom older version of Minecraft (without mods) results in a broken game state, preventing proper launching and causing a soft lock. I followed the instructions in https://www.reddit.com/r/MinecraftHelp/comments/tghe80/java_how_can_i_access_beta_16_test_build_3/.

Steps to Reproduce:

1. Navigate to the ".minecraft" folder and go to the "versions" folder.
2. Create a new folder called "b1.6-tb3" (without quotation marks).
3. Visit the following wiki page: https://minecraft.fandom.com/wiki/Java_Edition_Beta_1.6_Test_Build_3
4. Click on "Client" under the "Downloads" section to download the "b1.6-tb3.jar" file.
5. Place the downloaded "b1.6-tb3.jar" file into the newly created "b1.6-tb3" folder.
6. Right-click on ".json" on the wiki page, select "Save As," and save it as "b1.6-tb3.json" in the same folder.
7. Open the Minecraft launcher (close and reopen it if it was already open).
8. Go to the "Settings" tab and ensure "Show historical versions" is checked.
9. Click on "New installation," give it a name, and open the "Version" dropdown menu.
10. Scroll down to the section with versions starting with "old_beta" or "old_alpha".
11. Select "old_beta b1.6-tb3" from the list, save the installation and click "Play".

→  The game launches with the newly created version.

Observed Behavior:

Upon launching the game with the custom older version, Minecraft displays an error message stating it cannot run and becomes stuck in a loading state. The "Play" button remains unresponsive until the launcher is restarted, resulting in a soft lock.

Expected Behavior:

When launching Minecraft with the custom older version, it should load and initialize the game normally, without encountering any errors or getting stuck in a loading state. The "Play" button should function properly, allowing gameplay to proceed without requiring a launcher restart.