It is random, but I also just verified something of an inverse:
1) Opened new tab. Navigated to the ordinary Sign-in (also bookmarked):
https://account.wolfram.com/auth/sign-in
2) Did not sign in as requested. Opened new tab. Navigated to the App page, Mathematica Online:
https://mathematica.wolframcloud.com/app/
In this attempt, it showed HTTPS. However, I then started a response to your query, at which Sign-in was definitely required.
I am not certain there is a pattern to it; sometimes if I don't sign-in, but restart a new tab for the App approx. 2-3 times, it will ultimately switch from HTTP to HTTPS. I otherwise just Sign-in, but remain aware it is ongoing. Because I notice that sometimes in the course of heavy use, the App will arbitrarily switch from HTTPS to HTTP. I think when I actually open a Notebook to edit, that tab is typically secure.
Also, I am not an everyday user and haven't been on the platform much recently.