Upgrade problems Win10 IIS

IIS 10.0 on Win 10 machine. PHP latest.
New install - evaluating the product, looks promising!: grav-admin unzipped from getgrav.org. After all dependencies resolved/fixed, runs :), default UI, add admin user #1.
Prompted to upgrade 1.7.39 -> 1.7.39.1 via "Upgrade Grav Now" button: Squish.
Message of success in web UI. Nav to any Url, returns 403.13 (no default doc), including root of site.
Observe that index.php is missing in app folder. system/config/system.yml has been overwritten (lost changes to username_regex, pwd_regex).

Copied index.php out of original zip file, and added changes back to system.yml. Restart web server.
Seems to work, and am now presented with a new prompt to upgrade to 1.7.39.2.
"Grav v1.7.39.2 is now available! (Current v1.7.39.1)"
Squish button. Successful update.
index.php once again missing, system.yml once again overwritten.
Copy again from zip, update yml. Browser reload. Apparent success.
Footer says v1.7.39.2 - Admin v1.10.39.

The date on index.php is 1 month as of today (2/20/2023), so I'm sort of assuming no changes since 1.7.39.0.

I also have detailed missing steps for Win 10 virgin install if you want. Numerous things missing from doco. This is a dev machine, but I'm going to want to reproduce/copy to a public Win server.

Is a copy via git clone preferable to install from this zip? Thanks!

Reproduce the Windows 10/IIS upgrade using the zip installation and the “Upgrade Grav Now” flow. Inspect what happens to index.php and system/config/system.yml after each upgrade, and compare the reported success message with the resulting 403.13 response and repeated upgrade prompt. Done means upgrades retain required files and configuration and leave the installation at the advertised version.

Written by the indexing model from the issue text.