Prevent Mac WIFI from being deleted when I delete from iPhone
This exact issue has also been a thorn in my side, as I frequently do not want my iPhone on the office Wi-Fi network for one reason or another.

A loophole that has been working for me lately (your milage may vary) is to forget the network in question from the iPhone, and then rejoin on the Mac. Its probably a bug, but I've found that I get several hours with the iPhone on cellular and my computer still on Wi-Fi this way. The phone does, unfortunately, rejoin the network once iCloud Keychain figures out what is going on and syncs up.

The only permanent solution I have found so far is to disable iCloud Keychain completely on the iPhone. That way, since the iPhone will not share passwords with your other devices, when you forget the Wi-Fi network on the iPhone the Mac will be unaffected.

Since I quite like iCloud Keychain and use it frequently, I find this a less than satisfactory answer. However, it will result in the desired outcome here.