Placing a slab under a block, and placing a sign on top of the slab, will break the block above the slab. Placing a sign in the same location as a smaller block (such as a fence post) will break that block (the fence post).
Comments 2
Cleaning up old tickets: This ticket had been set to 'Awaiting Response', but has not received a response from the reporter (~3 months+) so is being closed as Incomplete. If you feel this is still a valid issue then please comment, or create a new ticket following the Issue Guidelines which includes steps to reproduce the problem.
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Please make sure to update your game, this was fixed in 1.16.200, see MCPE-102371.