Had been a running a stable May release until recently when I jumped to an Aug 10 prerelease (and several versions after). I believe something between those versions introduced a bug in custom channel playback. When playing a custom channel in Apple TV Channels, it says the content is in my library (see below).
The key is that these channels have no EPG data (no XMLTV and no m3u tvc-guide-stationid=). Ignoring the fact there is no match for these channels in the library, even if there was, how could Channels identify a match without EPG? Included the raw M3U below for reference.
#EXTM3U #EXTINF:0 channel-id="rewind9991" channel-number="9991" tvg-logo="",Rewind: CSI http://dvr.lxc.lan:8080/rewind/9991/miami.m3u #EXTINF:0 channel-id="rewind9992" channel-number="9992" tvg-logo="",Rewind: Cutthroat http://dvr.lxc.lan:8080/rewind/9992/cutthroat.m3u #EXTINF:0 channel-id="rewind9993" channel-number="9993" tvg-logo="",Rewind: Love Boat http://dvr.lxc.lan:8080/rewind/9993/love-boat.m3u #EXTINF:0 channel-id="rewind9994" channel-number="9994" tvg-logo="",Rewind: Hallmark http://dvr.lxc.lan:8080/rewind/9994/hallmark.m3u #EXTINF:0 channel-id="rewind9995" channel-number="9995" tvg-logo="",Rewind: The French Chef http://dvr.lxc.lan:8080/rewind/9994/french-chef.m3u #EXTINF:0 channel-id="demand9999" channel-number="9999" tvg-logo="https://cdn.iconscout.com/icon/free/png-512/on-demand-1780807-1514383.png",On Demand http://dvr.lxc.lan:8080/demand/9999.m3u