New
#11
Now that is weird if this was a clean install. It means that Setup must have read the key from the firmware in order to use it as the installed key.
Yes, Setup can tell from a key what edition it should install. Only if it doesn't know what edition to use would it ask for a key. I had assumed until now that it was a failure to read your key that caused the prompt, but @alphanumeric's theory that "the algorithm Microsoft uses to read the key was upgraded at some point" could mean that while it can still read the key, it can't determine the edition from it.
A test for that would be a clean install using old install media from a build that had previously not asked for a key. If it still doesn't ask for a key then it's a bug in the newer install media, not a hardware issue in your PC. I'm not suggesting either of you should try that immediately, but next time either of you need to reinstall it would be an interesting experiment to try.
I'm subscribed to this thread, so I'll see your results as and when you ever get round to trying....