I confirm that the bug still exists, Ubuntu Mate 18.04, dual monitor set-up (pc screen + projector screen via HDMI). Notify-send + notify-osd works perfectly when no window is in full screen mode (F11), doesn't work nor on primary or secondary when there is a full screen window opened (doesn't matter if is on the primary or the secondary display).
I confirm that the bug still exists, Ubuntu Mate 18.04, dual monitor set-up (pc screen + projector screen via HDMI). Notify-send + notify-osd works perfectly when no window is in full screen mode (F11), doesn't work nor on primary or secondary when there is a full screen window opened (doesn't matter if is on the primary or the secondary display).