Sorry. Perhaps my post was misleading. When I wrote “Same .m3u file (tvheadend links) is working using Windows” I was NOT playing the .m3u file with KODI. I was using VLC with Windows.
I can confirm that the same .m3u list does NOT work with KODI 19.5 in Windows (with previous versions it worked).
I can’t provide a public example, but I guess that all tvheadend lists (user:pass@host:port/etc) will have the same problem.
And yes… your workaround is working.
Replacing:
http://user:pass@host:9981/stream/channelid
with:
#KODIPROP:inputstream=inputstream.ffmpegdirect
#KODIPROP:mimetype=video/mp4
http://user:pass@host:9981/stream/channelid
(using Notepad++)
all the channels seems working.
A bit laborious, but it works.
Anyway, a fix would be good 
.