I have to take words back. In Xubuntu 16.04 and 18.04 system does not wait until xflock4 finishes before suspending, so the bug seems to be present in Xfce 4.12
I have to take words back. In Xubuntu 16.04 and 18.04 system does not wait until xflock4 finishes before suspending, so the bug seems to be present in Xfce 4.12