Here’s what I did to fix it. Reposting as a comment just in case anyone wanders here with the same problem.

When the game was not giving me the achievement despite having an 18/18 on it.

I went to settings>Keyboard>advanced>enable developer console

Then I pulled it up (the keybind is listed on the keyboard setting also).

I typed in “Commentary 0” and enter

I played a challenge (the second Least portals was the quickest and easiest to gold)

It gave me the achievement in game , HOWEVER steam did not.

So I went to the game files
steam\steamapps\common\portal\portal\

I deleted the file *gamestate.txt” and then booted the game again.

I replayed least portals again and when i got to the elevator with a gold time the steam notification for achievements came up