There is a lot of contention around having to worry about the distinctions between MPEG-TS and HLS streams in custom playlists.
Specifying the format as MPEG-TS causes the DVR to accept both formats.
Please update the documentation so the users do not have to go through an unnecessary exercise in order to figure out what the content of the playlist is.
Big Thank You to whoever made it possible 
It's up to you to know what streaming format your Custom Channels are coming in. IE, your provider tells you this. Either it's MPEG-TS or HLS. I honestly don't know what else we could possibly document.
If you don't know, and it doesn't work, simply editing your Custom Channels source to change the format, is available. So, if you do not know, it will obviously take trial and error.
MPEG-TS format should be renamed as 'anything and a kitchen sink' since it accepts everything. It should be the only one available and the entire option removed. The unnecessary availability of two choices for the M3U list format creates confusion among users and developers alike.
While the internal process may identify other formats, that does not obviate the need for the user to tell Channels which format the streams are in. (And as a follow-up, any user running into issues with Custom Channels likely does not have unique channel-id tags.)
What confusion are you referring to? It's A or B; if A doesn't work, then it's B. And if you're a developer, you should already know. (And if you don't, then you're not really a "developer", you're a shadow of a script kiddie.)