When inserting a wikilink in a Meta global user page, like [[Interwiki map]], it is a broken link in all local wikis.
As workaround someone could suggest to use something like [[:meta:Interwiki map]], but sadly it works in local wikis but not in Meta, because meta: is disabled in Meta :)
How to deal with it?
m: works.
I'm surprised to see that meta: is considered an interwiki link but indeed it is listed at https://meta.wikimedia.org/wiki/Help:Interwiki_linking#Project_titles_and_shortcuts - maybe the documentation is wrong?
In any case, not an issue in GlobalUserPage itself but with interwiki links?
The best practice is to always use https://meta.wikimedia.org/wiki/Help:Interwiki_linking#Portable_links
Re: Documentation, it does explain below the table that "Some of the long forms don't work as expected within the same project [...]". The same applies for things like [[commons:foo]], [[wikipedia:foo]], etc.
As per last comment I'm closing this task as invalid. If the documentation is not clear enough, please feel free and go ahead to make it clearer. Thanks a lot! :)