While placing a sign in adventure or survival mod, it's always pop up "Edit Sign Message" GUI. BlockEntityTag won't work.
Steps:
In creative mode, pressing ctrl+middle click on the mouse to get a sign with BlockEntityTag.
Change game mode to adventure or survival.
Place that sign.
Thank you for your report!
We're actually already tracking this issue in MC-105216, so this ticket is being resolved and linked as a duplicate.
However, that ticket has been resolved as working as intended, which means that this will not be fixed as it is not considered a bug.
Please be sure to use the search feature before creating a ticket to see if the issue has already been reported.
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