Finally! Thank you!
But, since Roku remotes generally have 3 buttons that could be considered a "back" button (2 different LEFT arrows, plus what I know as the "instant replay" button), I'm going to specify that the one I just used that finally worked was the TOP-MOST LEFT ARROW button that one might call a "back" button.
On MY remote, it sits at the top, in a row of only 2 buttons JUST below the POWER button at the very top. Of those 2 buttons, the LEFT button is of a standard ARROW WITH A SHAFT pointing LEFT (as opposed to just the "<" sign). THAT is the "BACK BUTTON" THAT WORKS in this scenario. (The only other button on that row is the HOME button, with an icon of a house.)
Last comment: I came here searching for a solution to this same issue in the Netflix app on a TCL Roku TV. So, this worked for me in Netflix, but I haven't tried it yet in YouTube. I'm crossing my fingers that it works there too, because this has become super frustrating in MOST apps since I bought this TV less than a year ago!
CLEARLY, this has been an ongoing issue that is DEFINITELY A ROKU ISSUE!
I have multiple other ways to access all the same streaming content ON MY TCL ROKU TV (Apple TV, PS4, "Smart" BR player, FireTV Stick, Chrome Stick). Plus, I can of course watch things on my PC, my Mac, my iPhone, my iPad, etc. We all have soooo many ways to check CS's claims when they start playing the finger-pointing game. If we can access the same apps via different platforms, and we don't have the same issues as we do when accessing that content with our ROKU devices...guess what conclusion most logical people generally come to!
HEY ROKU...PAY ATTENTION: When the exact same issues that YOUR CUSTOMERS (the ones that pay your salaries) complain about happen on EVERY APP WE USE on THE ROKU PLATFORM, we KNOW you're lying when you try to tell us it's the app developer's fault!
ROKU, please TAKE RESPONSIBILITY, and then FIX THESE ISSUES!!!
If you don't take care of your customers, you won't be around much longer.