I tore my hair out all Friday because my page picker would return broken links for the field if the user changed anything at all on the page.
Create a page, and “accidentally” put a trailing space after the page title.
On another page that uses the page picker, select the page created in Step 1. Once you Save and Edit, the link to the page will work fine.
Now go into edit mode on that second page (in step 2), and make any change to the page, save and edit.
In view mode, the link to the page created in Step 1 will be broken. Xwiki will have stripped out the trailing space (%20) from the doc’s fullName