The problem here is the authentication keys in the format example.
Format starts with a blank Mifare Classic card, with the authentication key set to 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF.
NDEF formatted tags use a different authentication key, which is written to the sector during the format process.
So ... when you try to rewrite to the same sector the sample format code uses the old authentication key which won't work.
I just added a new example that should allow you to update records on previously formatted card: mifareclassic_updatendef
Be sure to only use the next example on cards that have already been formatted via mifareclassic_formatednef or you will have similar authentication key issues.
You'll need to get the latest code from github, but you can see the source here if you are curious: https://github.com/adafruit/Adafruit_NF ... updatendef