I found that if a comparator is outputting a signal from the dispenser into a block that has redstone dust on top of it and then a block to the side of the redstone dust facing the dispenser direction, then any block placement update to the dispenser will trigger it to shoot out an item that it has stored.

View the following video for more in-depth visualization:

https://drive.google.com/open?id=1DwZtvPw393sysv_3HSl3JmUYFdnOm2GVhttps://drive.google.com/file/d/1DwZtvPw393sysv_3HSl3JmUYFdnOm2GV/view?usp=sharing

This seems to affect 1.15.2 and the newest snapshots, I haven't tested it on any earlier stages of the game.