It could be because you have LNURL in the lud06 Lightning address field instead of the lud16 LNURL field. I didn't confirm though. Am mobile at the moment.
Discussion
Found the bug. Essentially decoders MUST lowercase the prefix "LNURL" to verify the checksum.
The things you only learn in production...
https://github.com/vitorpamplona/amethyst/commit/310db1008ecc6691404882d67e47a3219fbc1375
Ok, will look into