On Sun, 19 Mar 2017 20:26:25 -0700, Jeff Liebermann wrote:

What you should be asking is what disables wi-fi
scanning. That's easy, 2 ways.
1. If your phone is setup to act as a hot spot so that others can
borrow your cellular data bandwidth, it will kill scanning. That's
because a hot spot requires that the channel number be fixed and not a
moving target.
2. If your phone is in peer to peer mode instead of infrastructure.
There are many reasons for this to happen. For example, printing
directly to an HP ePrint printer or Apple Airprint printer. I think
(not sure) that a GoPro camera connection does the same thing.

Note that the phone cannot be in infrastructure (what you want) and
peer-to-peer mode (what you don't want) at the same time.

Those are both good answers as to what disables wifi scanning.

The phone has never been set up as a hotspot since the last factory reset,
although it certainly would be possible to set it up as a hotspot since
it's T-Mobile which allows that on all their phones. But it's not currently
set up as a hotspot to my knowledge. But that's an interesting observation.

I'm not sure what "peer-to-peer mode" is for Android.

Googling
https://www.google.com/search?q=andr...r-to-peer+mode

I found this:
https://developer.android.com/guide/...y/wifip2p.html
Which says that two Android devices can connect in peer-to-peer mode (aka
WiFi Direct).

I have never used WiFi Direct so I don't think it's in peer-to-peer mode.
Besides, the phone connects to WiFi when I manually type in the (very long
complex) SSID.

It just won't find any SSID when I press the scan button.
I think it's time for a factory reset, which should fix the problem.