Observers with an item frame placed on the front of the observer, with or without an item, outputs a signal when the world is entered. This does not seem to occur when a block is already placed in front.
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Thank you for your report!
We're actually already tracking this issue at MCPE-29548, so I resolved and linked this ticket as a duplicate.
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Does this still happen in 1.14? Probably related to or same cause as MCPE-29548.