shawnhy
Hi,
This relates to the language and association settings in Joomla core in which after you login, it will redirect to the page you set in backend. However, if that page does not belong to the language being set for current user (you can check in User settings in backend) and has association with a menu item of another language on your site, it will redirect to that page.
In this case, the user 'joomlart' is set with default language on frontend (Chinese): http://prntscr.com/klzfsv
And the menu item page (after login) 'welcome-login' has association with another item in Main menu zh-tw so it happened as you see.
I just removed the association and it works fine now.
Kindly check.