User mentions list does not display custom avatars

STEPS TO REPRODUCE

1. Login as Admin
2. Set an avatar (upload a picture) on your profile
3. Create an user (U1) and set an avatar for it as well
4. Create another user (U2) without setting an avatar
5. Go to a page
6. Edit it in WYSIWYG mode
7. Type @ to display users mention list

EXPECTED RESULTS

All users from mentions list have their avatars displayed near their username.

ACTUAL RESULTS

Only users with no custom avatar set have their (default) avatar displayed (e.g. U2).

The avatar is not displayed for users who set a custom one (I've tested with .jpg and .png images).