Have you done the lost sector today?

I ran it yesterday. You haven't provided any information on why you think killing the two Overload Minitours prevents a player from getting platinum rewards. I'm going to expand on that and ask what you consider to be the rewards for getting platinum.

You know they change everyday right? Yesterday's lost sector isn't the same as today's? The reason I know is because I ran today's lost sector multiple times killing all champions and never got the prompt saying I got platinum rewards and didn't get any exotics so I figured there must be some kind of bug and decided not to kill the last 2 overload champions and guess what, I got the platinum rewards prompt and the exotic chest I was after so.. that's your info.

Yes - I know that the LS rotate on a daily basis. (when you made this post - it was Bunker E15 and the time this reply is now The Conflux on Nessus) As for the on-screen prompt not displaying even when killing all the champions in a run has been a glitch in the UI since they implemented the Legendary / Master LS into the game. Just because it doesn't show doesn't mean you did not achieve a platinum rating. As for the rewards, you do know that getting an exotic is not guaranteed even if you get platinum. All that platinum does is increase (slightly) your chances of having an exotic drop.