We have a number of buyers on our list that show up as no FB left by us. Not true!

We get reminder messages from ebay all the time to leave FB for them.

But when we try we get a message that says something like you have already left FB.

So looks like a similar glitch to what you are experiencing.

Looking on the buyer's FB we can see the FB we have left too.

We never forget to leave FB as its part of the process we use when packing & sending.

Also we use it as a quick confirmation that we have actually packed and sent the item and when.