I have 9 fully explored maps in a 3x3 square of item frames as the achievement says and its not popping. The 9 maps are 1:1 maps of areas outside of the 1:8 map that I spawned with.
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Thank you for your report!
We're actually already tracking this issue at MCPE-27720, so we will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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Can you show us an image?