Have a look here: https://letsencrypt.org/2018/08/06/trusted-by-all-major-root-programs.html
First paragraph says: "As of the end of July 2018, the Let’s Encrypt root, ISRG Root X1, is directly trusted by Microsoft products."
I have just checked my Windows 10 20H2 certificate store, and ISRG Root X1 is nowhere to be found. I also can't find it on my corporate PC.
Is this expected? I tried searching for this, but since X3 still works I guess not many people noticed.