Not sure about the TS/M2TS files, but I went through and changed the language flags on many mkvs last night and RMP on my Roku Ultra consistently picked up the additional audio tracks afterwards.
I will rip a blu-ray today and see what MakeMKV gives me.
UPDATE: So I ripped a blu-ray with three surround tracks, a DTS-HD, DTS, and Dolby Digital track. Straight out of MakeMKV, RMP only read the default track. I stripped the language flag metadata out completely - still just default track only. I changed the language flags to English, Undetermined, Undetermined, and it read two of the tracks. I changed the language flags to three different flags, and it picked up on all three. It certainly seems like, at least for mkvs, RMP is only able to see one audio track per language flag per video.
I can also confirm that RMP is still unable to read multiple audio tracks on mp4s, even with similar header information.
Setting it to undetermined worked for me also, thanks!
There is an App called Watchaid for Apple TV that does this. So it's far feasible. It makes use of deep linking. Works with all famous streaming services including a number of difficult to understand things like EpixNow and Paramount+.