As you can see, procmail puts a slash in the middle of the name, which of course is not allowed. This causes the rule to break, and the mail to be delivered to the default mailbox instead of the intended.

That looks like this procmail bug:

https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=62859

Upgrading to procmail 3.22 should fix the problem.

Paul