Page password not working – page still visible

I’m using Ninja forms on WP. I have created a Ninja User Registration form and added it to a test page which I have password protected – I’m at the testing stage.
When I try to use the form as a visitor to the website – it shows the password field, then the entire multi-part form. If I ignore the password field (leaving it blank) I can complete the form (with test data) and it registers me as a user and logs me in. This means the password protection does not seem to be working.
I have 4 similar pages, 2 with this problem and 2 with the protection working. Looking at Quick Edit, all 4 pages seem to have the same settings. I have cleared the history and the cache.

I’m a newbie, and now have no idea what else to try

The page I need help with: [log in to see the link]