ANSI_X3.4-1968 is actually just an ancient alias for
US-ASCII, so the originator of the OOO replies is doing something wrong. Your finnish characters are not displayable in US-ASCII, so the originator of the message is not choosing an appropriate charset when generating the new message Subject.
We
can go to the trouble of supporting ANSI_X3.4-1968 as an alias for US-ASCII, but then you'd still see '?' on ZCS as well. If you'd like to see that, please file a bug in
bugzilla!