It seems to work fine still, I have dispenser currently turning off portals on a realm with no issues. What I did have to do was extend the pulse. I went to slower clock (observer clock worked well) vs. the previous iston clock. It was too fast. So manually doing it with a button works (slower input), or changing your dispenser clock to be slightly slower is the fix.
Quick update. This was corrected with a realm reset (open/close). Not sure of the cause but anytime it's happened once the realm is reset it appears to fix it. I've seen on two occasions now so it's not fixed, but there is a workaround. No idea of the root cause however, despite testing/observations.