NOTE: This was reported before, but it was marked as 'resolved'. Since this is platform-specific, I think it is a different bug.
Every time I open Minecraft 1.12.0 on my Android phone, the patch notes appear (as opposed to only showing the first time after an update). On my Windows 10 PC, this bug does not happen.
Cleaning up old tickets: This ticket has not been updated recently (~1 year+) so is being closed as Cannot Reproduce. If you feel this is still a valid issue then please comment, or create a new ticket following the Issue Guidelines.
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