In multiplayer, only one had the wiki button top-left.
All other players had no button.

Huh. Would the one that had it have been the first player?

exactly

exactly

Damn. Ok. Thanks I know what the issue is then and I should be able to get it fixed soon

When making GUI stuff, i usually make it check if the button top left exists, and if not then create it, every time a player joins.
There are even cases where a mod destroys the button and one has no way of getting it back after.
So a check if it is gone to rebuild it is always good. A rejoin can fix such cases then.