Hi!
You are seeing the intended behavior that has been in UE4 since the very first moment when I wrote that tab-management system. In 4.7 we automatically open new assets in the main tab-well. This made the behavior much more noticeable.
The side-effect of opening assets in the main tab-well is that the moment you open the asset, the Level tab is no longer in foreground. Whenever a major tab is backgrounded we hide all the other tabs that are its “children”. If we did not do this, things would get confusing pretty quickly. For example, you could pull a details panel out of your Level Editor and then open a material in the same tab well. Now you would be looking at a material, and you would have an orphaned details panel that lets you change properties of something you cannot even see!
Making design decisions about this is really tough. There are so many valid workflows, after all.
To get a workflow more similar to 4.6, just pull out the asset into a new window. We will then spawn other assets next to the one you moved, and the Level tab will never be backgrounded. That way you can still see all of its child tabs.
-NickA