Hello,
I currently have 5 transactions under the "shipped and awaiting your feedback" section. Feedback has been left. Double checked with the buyers to make sure that the feedback was showing on their end. When I do go to try and leave feedback again I also get the "you've already left feedback for this transaction".
To me this is a bug. The 5 transactions I mentioned were multiple purchases. Wondering if it has something to do with that.
All the best