I've figured out Force local home directory on startup disk is automatically enabled and enforced when create mobile account at login is selected for AD join with mobile account.
It looks to me that if AD user account profile has Home folder set to a UNC path, it will be automatically mapped at the user login, in which case if the Mac is not on the corporate network, the user will get the error message "There was a problem connecting to the server ...".
Tested this workaround and it appears to be working:
Login as the AD user mobile account enabled
Open and unlock Directory Utility
View User in node /Local/Default
Search the user's name
on the right hand side, find OriginalHomeDirectory
change its value to <home_dir><path>/Users/username/</path></home_dir>
After this, when the Mac is offline, logging in as the user doesn't have the drive mapping error anymore and the home folder/drive can still be connected to manually.