@dredmorbius I think I've got it figured out: it's something to do with how MacOS interacts with Active Directory.

From cmd: 'ssh [domain\userid]@[host]' gives me a home directory of /var/empty, just as 'ssh [host]' does.

'ssh [userid]@[host]' gives me the correct home directory of /Users/[userid].

Cygwin must not bother with trying to auth against AD unless explicitly told to.