For webpages other than the homepage, Squarespace uses the Built-in Domain on mobile instead of the Domains Managed by Third-Party

I'm a website newbie and happy with the progress I'm making for a prototype site. Doing things on my own for the first time. I successfully attached a domain I own and thought everything was in good shape... but I just noticed that Squarespace doesn't use the domain for webpages other than the homepage on mobile web. It does on desktop. Has anyone seen this before? I would love whatever guidance you could give me.   :)

If you provide the domain I will take a look. 

Thanks so much, @derricksrandomviews. I am attaching a screen shot. And, just for example, I can go from https://www.doctorsorders.xyz/ to https://www.doctorsorders.xyz/endocrine-and-metabolic-disorders on my laptop... but when I use my phone, I go from https://www.doctorsorders.xyz/ to https://doctors-orders.squarespace.com/endocrine-and-metabolic-disorders and I'm not sure why it transitions over to the Built-in Domain.