Unable to sign into app on Flex box

Why am I not able to sign into the Starz app on my Flex box? It’s the only app that doesn’t allow me to sign in using my existing account, the only option on the Flex box Starz app is to subscribe and bill it to my Xfinity account, which is the

last thing I want to do since I already have a subscription that is much much cheaper. 
I’ve been trying to be patient hoping it would get resolved but it hasn’t. It’s the only app on the Flex box (that I use anyway) that doesn’t give me the option to sign into an existing account, none of the other apps give me this problem. 
I read all the forums and this has been an issue for years that Xfinity keeps turning a blind eye to. It’s the same issue people have been having for years and Xfinity absolutely refuses to acknowledge it or fix it. 2 years ago people were talking about the same thing and nothing was done. I spoke with someone from Xfinity earlier and they had no idea why it’s like this, seems like nobody knows. 

Can Xfinity fix this please!?! Reading other comments I guess this is also the case with AMC+ and Showtime.