WiFi network joining via QR code does not work (iOS 18.3)

I have a WiFi network which my iPhone joins without problem if I manually type in the password, but if I set up a QR code and scan that, the iPhone first asks if I want to join the network, to which I reply "Join" then almost immediately comes back with a dialog saying Unable to join the network "XXX".

I have scanned the QR code to check what it contains, and it definitely has the correct SSID and password enclosed into it, and, as I say, I can join the network by typing the password in manually, but it just won't work using the QR code.

The WiFi access point is a fully updated TP-Link EAP615-Wall, and the authentication mode is straight WPA2-PSK (AES). I have tried different wireless modes, switching Guest mode on and off and so on, but although the iPhone joins the network OK with a typed password, it just won't do so using a QR code.

Is the QR-joining functionality broken in some way? The iPhone is a 13 Mini running iOS 18.3, and I can't help but wonder whether this iOS version has some sort of bug in this area.