An email in HTML format does not display rewritten URLs by Click-time URL protection service.
If your email is being displayed in HTML format, it is possible to have the text display a URL or link that is different from the actual destination URL.
For example, both links were successfully rewritten by the Click-time URL Protection service, but you can only view the rewritten URL when you hover your mouse pointer over the link or inspect the links properties:
How to check: