"v=STSv1 ; id=1234;" is valid as per RFC.

[mateusz834]

The RFC 8461 defines:

   sts-text-record = sts-version 1*(sts-field-delim sts-field)
                     [sts-field-delim]

   sts-field-delim = *WSP ";" *WSP

   sts-version     = %s"v=STSv1"

So based on that the "v=STSv1 ;" should be considered valid. The RFC only states that:

If multiple TXT records for "_mta-sts" are returned by the resolver,
records that do not beginwith "v=STSv1;" are discarded.

Examples (inside array, number of TXT records):
["v=STSv1; id=1234"] -> valid
["v=STSv1 ; id=1234"] -> valid
["v", "v=STSv1; id=1234"] -> valid (second)
["v", "v=STSv1 ; id=1234"] -> invalid (all)