Even thou I said I was using iPad 4, but I am actually using an iPad 6, it is just not on there.
so, I built a 3 by 3 piston door and it sometimes works and sometimes don’t, and I will show you in the video below.
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Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-16371.
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Could you also provide photos or a video of the redstone setup of that door? It could be that the repeater timing was wrong or could be caused by MCPE-15793.