I just noticed that "Act on new/unread mail in this folder" option is not working anymore. I don't know since what version. When I change the value of the option and click OK, if I reopen folder properties the previous value is restored. The only way to change the value is through Settings-->Plugins-->New Email Notifier-->Folders
I am seeing the same behaviour with kmail 5.4.0
It doesn't work in 5.4.1 too.
Fixed in 5.4.2