Hi,
i recently needed to change the wlan mac on my laptop (Kubuntu 18.04). This used to work on that machine a few years ago. This time when I change the mac in plasma-nm, the connection simply fails. Journalctl shows no error messages, it simply tries to connect a few times without success. If I do establish the connection manually (shell), it works. Out of curiosity I gave gnome a try instead of KDE (same session), there mac change works flawlessly as well. 
Since I updated Kubuntu a few times, I tried the same at my office machine (fedora 28): Same thing: Mac change via nmcli works like a charm, as do the random Mac options. If I change the MAC-addr in KDE via plasma-nm, connection fails. Happens with both, encrypted and unencrypted wlan connections. 
Out of curiosity I tried the same with a Debian live Image (latest weekly KDE build): Same thing here. 
Regards,
Henning