You may have accessed your certificates by using Windows Run: certmgr.msc. However I found out if you access your certificates by doing a windows search for "Internet Properties", then clicking on the "Trusted Root Certification Authorities" tab, you will find ISRG Root X1 there.
Compare it to what you see in certmgr.msc, and you will find out that there is obviously and issue. When you open certmgr, you need to show "Physical Certificate Stores" and "Archived Certificates". You can do this by ensuring you're in the "Certificates - Current User" category, and clicking on view. From there click on options then enable the two check boxes underneath "Show the following".
You will find ISRG Root X1 underneath: Trusted Root Certification Authorities > Local Computer > Certificates.