Hi,
I've had the witch farm build based on ilmango's design that worked fine in 1.12 and forge 14.21.1.2387. I've recently stated playing again so I've decided to upgrade to the latest versions. I fixed the issue with observers being stuck on. That's when the observers fell into an update frenzy triggering the pistons constantly. I grabbed a fresh copy of ilmango's world and it turned out it suffered from the same issue. I've restarted the game with the vanilla profile and the farm worked as intended.
After a bit of trial and error I found a workaround. I replaced the power rail that is connected to the activator rail (by the piston from the clock) with red stone and it fixed the problem. Since vanilla doesn't need that change I'm assuming it's a problem with the forge. I've removed all the mods and I can reproduce it every time. I've tried both the latest stable and latest latest builds (14.23.2.2611 and 14.23.2.2624). Both had the issue.
Steps:
1. download ilmango's world
2. start it
3. switch the lever to turn on the clock
4. wait