You may be thinking of screen mirroring. But Channels has never supported AirPlay. (Their custom video player precludes supporting it.)
This is not related to AirPlay. This is regarding the play to feature from Channels via iOS/iPadOS.
No wifi mesh networks or anything of that sort? Absolutely nothing has changed or happened from when it worked flawlessly to when it didn't?
And if absolutely nothing in your end changed, can you recall which versions introduced the change? And since you stated you're running beta-pre-release versions, have you tried rolling back to previous versions to see if that addresses the issues?
(Including details is always helpful. Software versions, OS versions, etc. ... Also, you're running beta versions; does this also occur with the stable/release versions—Apple allows both to exist together on the system?)
Unfortunately I can’t pinpoint this to a specific version, date, etc. I really just wanted to report it in the hopes that possibly others have experienced this and it could be investigated. I’ll continue to test and see if I can find a reproducible pattern…
Hi please submit diagnostics from the client next time it happens and we'll take a look.
Ah, yes I was using screen mirroring...but the net effect is the same: the TV plays the video/audio, and my phone has ff/rew/pause/play controls and a TV icon in the middle of its screen
I just tested over here and can corroborate your report, I am experiencing the same — the first “cast to TV” command fails and the second attempt succeeds. It was not like like before.
I submitted diagnostics and hope this helps.
EDIT TO ADD: it seems to be working properly now for me so this issue is not consistent 
Could someone post a video of what they’re seeing?
Is the video player trying to start and it’s black, and you have to back out of it?
Or is just nothing happening at all.
Go figure, it’s working properly for me now. If it happens again, I will grab video. I did manage to send diagnostics though.
What I experienced was the same as Ahwman described in the OP. With the first attempt, the TV display goes black, as if the video is about to play, but it doesn’t actually play. On the phone, I then tap the watch button a 2nd time, choose AppleTV, and then it plays immediately. This happened for me at least 3 times in a row. Now it’s working fine.
By the way, navigating a Channels DVR this way has always been an absolute joy. Who needs PIP? 
Exactly what I’m experiencing. Thank you for corroborating and hopefully the devs can get to the bottom of this as I absolutely love this feature, especially in iOS shortcuts…
Just submitted diagnostics, hope it helps 
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This issue occurred yet again today and I was able to time stamp it at 12:01 PM EST on 02/24/24. I also resubmitted diagnostics. Hope this is helpful…
My experience thus far is that it happens after an extended period of inactivity. After a few attempts, it then it starts working fine, at least for a bit. Only for it to return in a few hours, like the next morning.
After more testing, I can duplicate this issue 100% of the time. In my case, it only occurs if I am currently tuned into a live TVE channel and then attempt to play content from Channels on my iPad or iPhone to the same Apple TV.
So if it’s not tuned in, it just works fine?
What if you’re watching a recording?
It works the first time in every scenario including watching a recording with the exception of live TV (TVE in my case). If I am tuned to live TV and use this feature, it goes to a black screen with no audio/video. The second attempt always works…
My triggers are different then, because the issue persists even when watching a recording. I tried it again just now. The first attempt to "cast" content from Channels on the iPhone to that same AppleTV just gets me the black screen, and the second attempt works. It happened twice just now. I've submitted diagnostics.
Has any progress been made on this issue, as it’s become a real hassle since I use this feature many times throughout the day and have to remember to execute it twice in order for it to be successful.
I've repro'd this and am taking a look this morning.
Thank you so very much!