Games may want to show UI during level changes, so existing UI on the viewport does not disappear unless the developer writes code to remove the UI when the level is changed.
If a level has UMG widgets, swapping to other levels (including the level you're already on) doesn't clear away the UMG widgets.
1: Open any project.
2: Create a new UMG Widget with a Button in the viewport.
3: Scale the button's alpha (under its Color and Background) to 50%.
4: In the Level Blueprint, create a 'Event Begin Play' node chained to a 'Construct Widget' node chained to an 'Add to Viewport' node.
5: Play the level.
6: Open the console and enter 'open Example_Map' or whatever the current level is called.
7: Note that another Button appeared on top of the original. Each time the Example_Map is opened again, the button becomes more visible because they're stacking on top of one another.
Head over to the existing Questions & Answers thread and let us know what's up.