Looks like bip174 doesn't specify string encoding (utf8, ascii) for bytes identifier field of PSBT_IN_PROPRIETARY key.
Now how a psbt lib can check if the bytes identifier is a a valid string bytes or not?
Otherwise, any non-string bytes can also be stored in bytes identifier.
Ref https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki#proprietary-use-type