Thank you for contacting us.
I assume you are using a passwd command to change the password. The command indeed does change only the SSH login password, not the WebUI. I will report this to the development department for more details.
Blow is the feedback from RnD:
In later firmwares we separated root and admin users (with ssh you use root and with WebUI you use admin). When changing password in WebUI, the password is changed for both root and admin. If you want to change the password in terminal, when executing passwd command, you must give an argument of username (root or admin) e.x. passwd root and later passwd admin.