Applying what I know about WiFi and radio in general - the only thing that makes sense is that, God only knows why, your WiFi chip may not be seeing the required antenna load. (i.e. the antenna may be broken/not connected.)
That would absolutely cause the problem you’re seeing. As far as the Pi and the software is concerned, everything is Hunky-Dory because many WiFi implementations don’t report reflected impedance or SWR - or if they do it’s a pain to get the info. (Or, the device using it doesn’t really care.)
You got my curiosity so riz, that I actually took apart my 'bot so that I could take a really good look at the Pi’s PCB.
Looking at my board I don’t see anything that strikes me as being an obvious antenna. I grabbed two pictures of the Pi-3B (not the B+) from Sparkfun’s site here.
The small beige part in the upper left corner - marked with an arrow and circled - looks like some kind of ceramic antenna. I’ve never seen a ceramic antenna element before, but this might be one.
This is the back of the board, (also from Sparkfun), and the circled area may well be the radio. The very shiny IC (on the left in the circle), is probably the radio chip. It’s interesting because usually the radio is either in a metal cased IC, or is in a metal shielding can. The item to the right appears to be some kind of ceramic device for matching the radio to the antenna - since it’s “output” leads directly to a through-hole to the ceramic antenna on the other side.
Guess what! It looks like I was right!
MagPi, Issue 43, talks about the “new” Pi-3 and has some nice illustrations of the ceramic antenna and the radio chip.
Look at the devices around those and see if anything looks damaged or missing.
Let us know what you find.