When using a configuration seen in the attached screenshot, hitting the noteblock powers all four droppers but only on the first hit; all subsequent hits result in only 3 of the droppers being powered?!? The dropper that does not fire is the one next to the upward facing observer. Breaking and replacing this upward facing observer allows it to work once again (breaking after one use again)...

Sorry if this is some really niche intended logic, but it feels pretty buggy to me.

I tested this with dispensers as well with the same result; I also tested with redstone lamps but interestingly this does not occur with the lamps.