Summary

On a simple and AT sites, when any setting is changed in Advanced General, the theme breaks and completely reverts to a different theme or just shows broken/blank homepage for the site. I tested with Karuna and Twenty Twentyone active and after saving some new sittings in Advanced General, my test site switched to Twenty Fifteen.
WP-Admin/themes shows an error: The active theme is broken. Reverting to the default theme.

Steps to reproduce the behavior

1. On a simple or AT site, make sure one of our internal themes is activated (such as Twenty Twentyone) and then save any setting in Settings ➤ Advanced General
2. After saving the settings, the theme is broken and reverts to a different theme. For example, I had Twenty TwentyOne active, and that was reverted to Twenty Sixteen.
3. WP-Admin ➤ Themes has an error: the active theme is broken. Reverting to the default theme.

Is this specific to the applied theme? Which one?

Seems to happen in both block-based themes and premium themes.

Does this happen on simple or atomic sites or both?

Simple and AT, but so far only tested themes in our internal library