Joan
Comments
I can confirm this setup was working on 1.14. Seems to be something observer-related and not a hopper/dropper/dispenser issue, since those blocks work fine when there's no observer involved.
I can corfirm the bug happening. To add more info, updating any block adjacent to the hopper that is next to the dispenser/dropper will make it work as intended for ~2 seconds, and placing any item inside the dispenser/dropper will make the hopper work only until it depletes.
It's not working as intended, the clock powers the block, but only for a tick, so there's no reason to stop the hopper from placing items inside the dropper. That said, there are already some people who posted this issue (MC-151815, MC-151418...) and seems to be something related with the observers powering blocks in a way they shouldn't.