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Sounds like you didn't have the ACNL puzzle and you didn't get the spotpass data for the new puzzle when the new piece was being distributed. So you got a random piece from a puzzle you didn't have instead.
This happened to me when the LMDM puzzle came out, I got a Mario Tennis piece instead.
"In truth, they haven't been allowed for ages, we've just never enforced the rule" -SineNomine on presigs