Is mnemonic words -> mnemonic sentence standardized?
From bip39:


  To create a binary seed from the mnemonic, we use the PBKDF2 function
  with a mnemonic sentence (in UTF-8 NFKD) used as the password and the
  string "mnemonic" + passphrase (again in UTF-8 NFKD) used as the salt.
  The iteration count is set to 2048 and HMAC-SHA512 is used as the
  pseudo-random function. The length of the derived key is 512 bits (=
  64 bytes).


When normalizing unicode in NFKD form, characters that look the same become the same, so U+3000 becomes U+20. This may also cause length to change as composite characters are broken down into their constituent parts.