How do I complete a sideways encounter?

In Fortnite, we’re currently in chapter 2 season 8 and I’m stuck on a mission for Torin to complete a sideways encounter:

According to my research, to complete a sideways encounter I need to defeat three waves of cube monsters within a set time limit that begins when I enter the sideways:

Sideways encounters in Fortnite involve fighting off waves of Cube Monsters until you fill the orange challenge bar at the top of your screen. If you do this successfully, then you will complete the Sideways encounter. There is, however, a catch and it comes in the form of a timer.

Upon entering the sideways, a bar is supposed to appear at the top of the screen:

However, I’ve dedicated almost 30 matches solely to accomplishing this mission and have not once seen the bar. I have however noticed three circles at the top (I’ll try to get a screenshot soon) that appears when (as I know them) the cube bosses do:

However, defeating them (which is a pain alone by the way), doesn’t complete the encounter, so I’m missing something.

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How do I complete a sideways encounter?