Copied from https://www.mediawiki.org/wiki/Topic:V3b53ed1gg49pct4. It is related to T103635 but not entirely the same problem.

Steps to reproduce issue:

1. Open a page in VisualEditor
2. Type some text
3. Highlight the text
4. Click on the link icon to turn the highlighted text into a link
5. Type a page name with an interwiki prefix into the link destination box, e.g. :de:Haribo
6. Click "done"
7. (Optional: Click on the link text and then click "edit" again to see that VE has already stripped the leading colon. You can add the leading colon back at this stage and VE will strip it again.)
8. Save page
9. Upon saving page, the new link becomes a sidebar interwiki link, not an inline interwiki link.

The difference between this task and T103635 is that in T103635 the user hasn't prefixed a colon and we want VE to guess that the user wants an inline interwiki link. But in this task, the user has deliberately prefixed the colon to signify that they want an inline interwiki link, and VE still strips it and turns it into a sidebar interwiki link.

This problem affects any attempt to turn an existing internal link into an interwiki link, or any attempt to turn existing non-link text into an interwiki link. On the other hand, if one creates a new link without highlighting existing text, VE parses a colon-prefixed page name correctly and gives an interwiki link.