If the sign is in chunk 1, and the block that the sign rests on is in chunk 2, the sign will pop off over time. I havn't been able to recreate exactly but I believe this is happening when you leave the area at the distance that chunk 2 stops being in the loaded area.
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-11477.
It has been linked to this report. If you have additional information, please add it to that report.
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