I have the same problem that ONLY happened about 2 weeks ago.
Prior to that, the Roku app on my iPhone 14 would allow me to stream / watch ANY Roku TV channel (Live TV or Roku Channel) on my phone. Now when I try to actually watch a channel, the app turns the actual Roku player and my TV in the living room on and I "may" get the channel to open on my phone.
Plus, the app insists on controlling my Roku 4k in the living room. If I can get past that, which hasn't happened now for 2 or 3 days, I could pull up the channel guide and see the channel listings but not see the show playing.