Observer block stays powered indefinitely

In my sugar cane farm, I have row of 8 observer blocks facing downwards with redstone dust on top. These observers are supposed to detect when a sugar cane reaches a height of 3.

After some time, I came back to the farm and all the observers were stuck in a powered state.
Removing the surrounding blocks and the sugar cane did not change anything.

The only workaround was to remove all the observers and place them back.