I did workaround filters not working for inbox¹ by configuring server-side filters. Now, the problem: every time a new mail appears, KMail pops up a notification even if the mail is marked as read (i.e. on the server).

Steps to reproduce:

1. Create a server-side filter that marks mails with "test" as a subject as "read"
2. Open KMail, and send yourself a mail with subject "test"
3. When you see the mail appeared in "inbox", wait for around 10-15 seconds for a notification.

What happens: a notification pops up about a new mail "test"
What should've happened: nothing. The mail is read, there's no need to notify about it.

P.S.: My KMail is actually 5.6.2, but latest version I am allowed to choose here is 5.6.0.

1: https://bugs.kde.org/show_bug.cgi?id=378231