Thanks for the quick response @BarryCarlyon you do great work here.
Unfortunately, adding the trailing / did not change anything, I waited 5 minutes, re-copied my client id/client secret just to be sure and was not successful.
the redirect url in all 3 places is now:
https://mydomain.com/twitchcallback/
https://mydomain.com/twitchcallback/
https://mydomain.com/twitchcallback/
I realize I am most likely just missing something obvious, but is there anything else you can think of? I will continue testing and report back if I find a fix. Again, thank you for your response.