This appears to be by design to address compatibility issues, discussed here - New iOS Plex Experience - Live TV wants to transcode instead of direct playing - #10 by t0mm1pl3x
My read is Digital TV can be broadcast with encoding leveraging many different codecs, here in Australia DVB-T content is a mix of MPEG-2 and MPEG-4 (AVC/H.264). Understand that in other markets DVB-T2 is available that will take advantage of HEVC encoding in some cases.
Given this I can understand how Plex might want to, for a hopefully very short period of time, remove the variability involved to stabilise the Live TV product. However if I were in a position of having a NAS that can’t transcode in hardware, I can understand how this might be very annoying.
