SharePoint /:f:/r/ links fail from Outlook: %2F becomes %252F

We are seeing a reproducible issue with SharePoint Online folder sharing links (/:f:/r/) sent via Outlook.

A freshly generated folder link works when opened directly in Edge. When opening the link from Outlook, it can also work the first time, but subsequent attempts fail with:

"Unknown render failure. This item isn't available."

The failing URL shows what appears to be double URL encoding in the SharePoint id parameter:

Working:

id=%2Fsites%2F...

Failing:

id=%252Fsites%252F...

We tested:

- Edge and Edge InPrivate

- Fresh authentication

- Normal paste into Outlook

- Hyperlink via Ctrl+K

- Microsoft Defender Safe Links configuration

A normal SharePoint site URL sent via Outlook works correctly.

We also found a reliable workaround.

Instead of the SharePoint sharing URL:

https://<tenant>.sharepoint.com/:f:/r/sites/.../Forms/AllItems.aspx?id=...&share=...

we use the direct library URL:

https://<tenant>.sharepoint.com/sites/.../Forms/AllItems.aspx?id=...&p=true

The direct AllItems.aspx URL works both directly and when sent through Outlook.

Has anyone seen this behavior recently? In particular, why would a folder sharing link work on the first click but subsequently result in %2F becoming %252F?

We have already opened a Microsoft support case.

Note: This post was written with the assistance of Microsoft Copilot based on my own troubleshooting results and tests.