I've updated from Confluence 5.7.5 to 5.8.15 over the weekend, and the favicon reverted to the default. Looking at the settings, it's still showing my icon. Looking at the page source, the linked-to icon is the default.
On JIRA I have similar behavior.
Yes, I've tried forcing a refresh on my browser. I've also cleared the cache on our reverse proxy (nginx).
So silly...
Edit: Also tried reverting to the default icon, then re-uploading ours.