Updated description by GoldenHelmet

Steps to reproduce

1. Place a on observer next to a piston and connect it to a redstone circuit.
2. Place water or lava on the opposite side of the observer from the piston, or break a block next to a water or lava source so that water will into the block on the opposite side of the observer from the piston.
3. Very quickly after step (2), activate the piston to push the observer into the newly-placed/generated water/lava.

Expected result

The observer pusles.

Actual result

The observer lights up and never unlights, and becomes unusable.

Video demonstration (from 1.14)

Original description
Not long ago, I find my fly machine is not wroking, because when an observer block was observed the water block(quartz stairs), The observer block like stopping work.and texture is displayed it was constant activated.

This is not 100% like this.

Maybe my save can help you.

Happy new years.