This mainly happens because some browsers do detect if a webpage with the same TLD name exists in your current session and reload the new URL in the same tab.

I have never seen a single browser that does that. It sounds nightmarish.

If I visit app1.domain.com and then also try to open app2.domain.com, it would just open it in the first tab?

Or if it only works for the same origin - then I cannot open two questions from Stack Overflow?