This may be related to bug 42986: VisualEditor and Parsoid autoconvert URLs in the external link format to local links, including for interlanguage and other interwiki links.
This means that if you paste https://outreach.wikimedia.org/wiki/Example in as your link on one of the Wikipedias, it gets saved as [[outreach:Example]].
This is fabulous and should be kept.
However, if the local wiki doesn't recognize the converted prefix for the link, then the result is a redlink. At the moment, office.wiki doesn't know about the existence of outreach.wiki, but links are converted anyway.
The result is that pasting https://outreach.wikimedia.org/wiki/Example as a link on office.wiki gets saved as [[outreach:Example]], and when you click on the resulting redlink, it takes you to the non-existent page https://office.wikimedia.org/wiki/Outreach:Example
This is not so fabulous.
The most flexible system is to find out which wikis are known locally, and only convert those. The most immediate solution is probably to tell office.wiki where to find outreach.wiki.