User Mention does not work on document-type attachments pinned comments

Summary

On document-type attachment (such as word document), user mention does not work one pinned comments

Environment

So far observed on the following type of attachments

• docx
• xlsx
• pdf
• pptx

Steps to Reproduce

1. Create a page, attach a .docx file (example: testword.docx )
2. Save the page
3. Click on the attachment to trigger the view file window
4. Click the "pin comment" icon that says "Drag this pin to add a comment" from the bottom of the page > place the pinned comment anywhere
5. A window with comment field will appear. Try to mention another user by typing "@"

Expected Results

Dropdown user search will appear. Users will be able to pick the user that they want to mention.

Actual Results

Nothing appears.

Nothing logged in atlassian-confluence.log

Workaround

Refresh the page and the user mention should work again after.

Notes:

• This feature was seen working in 6.5.1