For Plex, these two files are the same “movie”.
Therefore it offers you to continue watching it for either file.
Say you start watching it at home in 4K.
Then you stop it, go outside with your smart phone and want to finish it while on some public WiFi hotspot. It is sane to choose the non-4k version then, because your server would likely choke on the 4K file when it tried to transcode it down for your smartphone.
So it is a feature, not a bug.
You could avoid this behaviour, if you set the primary metadata agent for your 4K library to TheMovieDatabase and keep the agent in your regular movie library at the ‘Plex Movie’ agent.
If the same movie gets ‘matched’ by different agents, Plex will consider them as different movies.
