Hi @675mrdee and welcome, is there a specific reason why you didn’t want to install the flathub version?
Regarding the MKV playback, it’s perhaps good to note that MKV isn’t a video format but rather a container. Inside the container are the actual media files which can consist of a variety of different codecs. So it may depend on the contents of the MKV which plugins are needed in order to get a working playback.
Reasons are mostly immaterial. The rpm version works well without the overhead of a flatpak and the necessary runtime packages. Users are free to choose without needing to justify their decision.
Your comment about the mkv container and required codecs is spot on. The codec issue is mainly what is managed by doing the group upgrade of the Multimedia group.
Just as an aside comment.
I use totem (videos), mpv and vlc among other multimedia tools and do not have any flatpaks installed to support multimedia. I am able to play all forms of multimedia I have encountered to date.