I use fastmail.fm IMAP service (free)

When I set up a filter to move mail from Inbox to some other folder, and I get 1 piece of mail, the mailcount says

Inbox 0 (2)
+- folder 1

So, when I get 8 pieces of mail that filter to folder I get

Inbox 0 (16)
+- folder 8


The count seems to be doubled. If I go to inbox and compress it, the count goes to 1.

The filter is "recipient contains <address>", Action: "Move to folder \\account\Inbox\folder"

thebat 3.9
Is it known bug? Is it repeatable by others?