Eu não vi esse comportamento antes, mas olhando para o man ssh_config
eu vejo:
A pattern-list is a comma-separated list of patterns. Patterns within pattern-lists may be negated by preceding them with an exclamation mark ('!'). For example, to allow a key to be used from anywhere within an organisation except from the ''dialup'' pool, the following entry (in authorized_keys) could be used:
from="!.dialup.example.com,.example.com"
Talvez isso funcione:
Match Group sftpusergroup User "*,!sftp_user,!sftp_user_2"
colocando toda a lista de usuários entre aspas.