I have the same issues on Samsung Q8F series.
I use to have a workaround that worked. Sign in/Sign Out TWO TIMES, and then all the HD channels will re-appear.
If that doesn't work you'll need to contact Xfinity Support to resend signal to your account and they will give you some bogus excuse that it is related to the TV app losing communication to your modem which is not accurate at all.
FYI.. you will go through this circus multiple times a month.. I'm 3 months in with no permanent resolution