Hi,
By default when binding to AD the directory utility forces the Local Home directory to be on the Startup Disk.
You can change this by logging into the Mac > System Preferences > Users + Groups > Login Options. Select Edit on "Network Account Server" then Select "Open Directory Utility" Unlock the padlock and then select "Active Directory"
Show advanced Options and untick the box that reads "Force Local Home Directory on Startup Disk" it should then default the home directory to your AD specified folder when you next login.
It's a well hidden menu for some reason! Does that fix your issue?