User mention or local link results in link to page it was created in

Scenario

1. Create page.
2. Set name of page.
3. Add user mention (or link to local confluence page) to page.
4. Click safe.

Problem

All added links change to links to the page they were added to. See the following screenshot.

Expected Result

User mention or link to local page is set correctly.

Additional information

Links to external pages are not effected. E.g. a link to https://www.atlassian.com remains a link to the Atlassian website as it was intended.
The problem appears to be browser independent. I tested it on Chrome, Firefox and Internet Explorer.
I patched from 5.7.1 to 5.7.3 this tuesday. On 5.7.1 we hadn't hat that problem but we also changed the address at we can access our Confluence at the same day we updated to 5.7.3.