A 12 word and a 24 word mnemonic both result in a 64 byte seed.
I'm surprised that you can get from that back to the mnemonic. I haven't managed to do so with the lib I'm working with.
Discussion
Quoting a fellow pleb: "Bip32, which is used to generate deterministic wallet starting from a seed, makes use of PBKDF2 with sha512 as pseudorandom function. This makes the the process not reversible."
TIL .... shit still learning Bitcoin basics