I've come accross a bug where an edit option is available or not based on a combination of permissions and where the report is accessed from.
I have a number of reports that are posted to a group's workspace. Members of the group have full rights to any reports in that workspace. If a member opens the report from the workspace, they can edit the report. If the same member favorites the report and opens the report from their favorites, they can edit the report.
If that member is given permissions specifically to their user account through the share feature, read and reshare access, they can still edit the report if they open it from the group workspace. If the same member opens the report from their favorites, the edit option is no longer available.
It seems like the report, when accessed from favorites is defaulting to either least permission or user permission and disregarding any permissions granted from group membership if user permissions are defined.
Let me know if screenshots or additional informaiton would be helpful.