Regardless, since 1.31 doesn't contain any Reforged files, including the new models, it's literally
impossible to experience the issue you described without updating to 1.32+. This means that either you messed up with the installation somewhere and you allowed the game to update or you have a separate Reforged installation.
The best tip I can give you right now is - just check how big your W3 folder is. I can bet it's going to be a lot bigger than 2-3 GBs, which means that you updated to 1.32. In this case, you have to uninstall the game, remove all game files and registry keys, and do a fresh install of 1.31 following the instruction above and some additional tips below to make sure that the game doesn't update.
To prevent updating:
- turn on the Battle.Net app and turn off automatic updates before installing 1.31 (details on how to do that are in the original post), this should work, but there's a chance that something will bug out and the app might start the update - just to be sure, the first time you launch Battle.Net app after installing 1.31, instantly click on W3 on the list of games and if you see that it's updating, immidiately pause the update and keep it that way
- launch the game using the .exe file listed in the instruction
- don't try logging in to battle.net from inside the game
- if prompted that an update is available - click "no"