As the title says, if you disable the friendly nameplates (Shift+V) and reload, the friendly nameplates are turned back on.

I can't reproduce this, then. If it doesn't happen with only KNP enabled (I realise it's annoying to do this if you have a lot of addons) then another addon is doing it. Nothing in KNP controls visibility apart from that option and the two drop-downs in "general", but they won't be taking effect until combat starts or ends.

I have a script for that in my ElvUI edit to disable all addons, so no problemo :P I'll let you know

Ok, i found the culprit. It is AdvancedInterfaceOptions

What situation was this in? On a boss, in dalaran, ...?

In every situation. A simple reload turned it back on.

If you disable /knp > boss mods > "allow boss mods to control visibility..." does it still happen?

Yeah, still the same issue.