Has your wife yet moved to iOS 14, or is she still on iOS 13? Judging from other threads here on the forum, I think I have a better answer for you.
Ever since I got my first iPhone, Apple complained that my WEP network had "weak security". Since I have a few legacy devices that can't do "better" than WEP, that's what I use in one of my networks. It appears that from iOS 14, Apple now also believes that WPA has "weak security". This could explain why your identical phones behave differently.
My original comment stands. If all of your devices can handle tighter security, do it. If you are worried about spies, tighten your security by tighter encryption. If you are worried about your neighbor stealing your WiFi, turn on MAC Address filtering in your router and register all of your family's own devices on the white list. If you are satisfied that the level of encryption you use is the right balance of security and accessibility, keep it.