Observers can detect block state changes like a piston moving, or when non full blocks like dust, repeaters and torches are powered and depowered, however the only block that doesn’t change states and can be detected as powered is the dispenser (which is why to me it’s a glitch and not a parity issue)

What should happen (ie: what happens with dispensers)

• The block is powered in any form, soft or hard powered
• The observer dectect the “powered” state and sends a signal

What happens for every other block

• The block is powered
• The observer take no notice

The clip below shows an array I used to test blocks. The pistons in the back tell when the observer passes a signal