Ok Zyxel, you win
Just got reminded of this old thing I had laying around...
Yes, I know, TRWTF is me not trying
That is why wars start.
I saw similarly brain-dead code with the modem/router that Comcast (briefly) leased me. You could configure the Wi-Fi parameters exactly once. After that, every attempt to change something would fail with a bogus error complaining that your WPA2 passphrase (which wasn't even changed) was invalid somehow. In order to change the Wi-Fi parameters a second time, you would have to reset the entire Wi-Fi configuration - then you could change the parameters once (better get them all right before you click the OK button) without another reset.