If you try to summon/setblock/fill a sign with pre-determined text, the sign simply displays as blank with nothing on it. Any of the special values, like setting a line on the sign equal to a players score doesn’t work either.
An example is provided below (both sides were empty):
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We're tracking this issue in MC-263051, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as invalid because as of 23w12a, the NBT format of signs has changed. Please take a look at the parent ticket (MC-263051) and see the explanation provided in the comments for why this issue is invalid.
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