I figured out the issue. This is caused by my manual migration from vpopmail user account. I imported user account from vpopmail into 'mailbox' table, but forgot to import into 'alias' table at the same time. After create users in alias table, everything okay now.

I cannot reproduce this issue with either iRedAdmin-Pro-MySQL-1.5.1 or the latest development edition.
Do alias members you want to add exist? iRedAdmin-Pro will verify submitted alias members and discard non-exist ones.

I need to setup an alias e-mail address (e.g. dummy@domain.com) so that if user send e-mail to this alias account it will auto forward to to all users (apple@domain.com, john@domain.com, paul@domain.com... etc.)
I created an alias "dummy@domain.com", now when I add member, e.g. apple@domain.com, and then save changes, it doesn't add to member list. But if I add other domain e-mail address (e.g. apple@yahoo.com) and save changes, it added successfully.

I add alias member by entering e-mail address in the "Add new members" textbox, and follow the rule "One mail address per line". Once I click the save changes button, e-mail addresses have just been inputted were gone without saving. I have to get inside to the database and add e-mail manually separated by a comma. Then I back to admin-pro and can see e-mail addresses (I add manually to db) are shown there inside "Members" box.