Below 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.
Best regards,
Žygimantas