Server bug: some pressure plates not working but others are

I'm running a Minecraft Java server (Linux) and for some reason, some of the pressure plates next to doors and gates are not functioning anymore, whereas others are. They make the noise when stepped on but the doors / gates don't open. And yes they are right next to the door/gates and worked fine until recently.

The server logs show nothing relevant, restarting the server does not fix the problem, and destroying the plates and then replacing them (or using new ones) doesn't fix it either.

Anyone got any ideas? It's really frustrating