Er… I’ve been sharing the ASCII value I entered into DT NFC since the beginning 
I used “ABCD” (ASCII, uppercase) which translates into 0x41424344. I used that value with all the tags I tried, failed or survivors. The survivors authenticated using that value - until they stopped authenticating, that is.
Specifically, with the tag in this Taginfo dump, I got “transceive failed”. It does not authenticate with 0x41424344 or 0xFFFFFFFF or anything else I’ve tried. Or rather, it times out.
I should also mention, for the sake of completeness, that I did try to use lowercase “abcd” (0x61626364) and variations thereof (“Abcd”) and it didn’t work neither.