Place blocks like the picture.
When opening the trap door, the upward piston should be powered.
The first piston might be a feature because the time is short, however, there's no reason the second piston doesn't get power.
Update: 1.11.2 the first piston be powered but the second piston still doesn't get power.
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[media]
Back in 1.13-pre4. Though it seems intended since MC-131805 was fixed.