I came across this bug while creating an igniter for a 3-motor flying machine. Refer to the attached screenshot for what I think is the simplest reproduction.

Expected behavior: the redstone wire should not have ANY effect on the piston. It can't trigger the observer, it can't cause block updates, and it can't provide power.

Observed behavior: If the piston receives a block update, then it treats the redstone wire as a power source: if the wire is powered, then the piston will stay extended; if the wire is unpowered, then the piston will retract.

If the observer is removed, then the piston stays on after receiving a pulse but will retract at its next update without another pulse.