If you update an observer either by moving it or a block update occurring in front of it, the observer will continue to power the block behind it indefinitely. To de-power the block you must brake the observer. This bug could brake almost all Redstone based builds since a large majority of todays contraptions rely on observers.
Thank you for your report!
We're actually already tracking this issue at MCPE-100302, so I will resolve and link this ticket as a duplicate.
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