Board Thread:Support Requests - Getting Technical/@comment-6301973-20200803000721/@comment-9605025-20200804045145

Juper0, the XML does contain information about the source wiki. However, an interwiki prefix is custom to the destination wiki. Therefore, when exporting the pages, it is not possible for the source wiki to include that in the XML since it doesn't know what it is. It is also true that the destination wiki may not actually have a prefix for the source wiki. I suspect that is why the import interface asks what prefix the user would like to use rather than actually checking to see if the source wiki is in the interwiki map. But that is just speculation.

Fandyllic, it is an "issue" with importing to a UCP wiki. The fact that the source wiki is a legacy wiki has nothing to do with it. Even so, the largest part of the "issue" is simply that it isn't clear to users what this field is for and/or why it is required. As Sophiedp pointed out, the workaround can be quite simple; depending on how the community wants to deal with it.