Hello Antoine,
I tried that developer link, but it is not working for some reason. I did find an archived copy from April 2020 on
web.archive.org (for anyone else who wants to help).
I did look at the script and am unfamiliar with Google Ads, but I can see what the script is doing. I would try playing around with how you enter your failure strings on the sheet.
For example, try Out of stock and "Out of stock" or just Out of or just stock, etc etc.
Also, it looks like it is grabbing the html body text. So if you have any html comment, keywords, metadata etc within the <body></body> of the page, it may be giving you a false positive.
Sorry I am not directly experienced with this Google product, but hopefully that gives you some ideas. I would also experiment and put a keyword on a page somewhere (same font color as background so it is not visible), and see if the script finds it. If you have a large site with many urls, then this may not be feasible.