[1.12] Redstone/Observer/Piston - Observer detects. Redstone lights up. Piston does not fire

I have a setup with observers attached to redstone for an automated sugar cane farm. on state change, the redstone lights up behind the observers. but the pistons below them do not fire...

Came here searching for bugs that might cause this.

The world is upgraded from 1.12-pre7 and many snapshots prior to that.

All player data/stats/advancements were reset on upgrade. (manual)

This is what it looks like