I think I know the issue. I just tried from my mobile device and I was able to access this private form and submit without ever being prompted to login. So that is the issue. I checked the settings and the form is not set to public but only shared with a handful of users.
I will create a test form and see if I can reproduce it then share the link.
Just to ensure that there is clarity on what you need to do from the post from @Kal_Lam,
You must check/select the Require authentication to see forms and submit data which is present under the ACCOUNTS SETTINGS (as shown in the image below). [Image 2.png] When this is done it will prompt the user to enter KoBoToolbox credentials again.