Any idea?
LE 9.0 uses the legacy vendor kernel and amcodec in Kodi. LE 10.0 onwards uses the upstream kernel which reimplements hardware decoding (but the implementation is incomplete) and Kodi dropped support for amcodec in 2017 as part of an intentional purge of prioprietary codecs and refocus on standards.
TL/DR: the codebases are completely different and while most media plays reasonably well on a C2 it is not pefect and will not work with all media for all users. The upstream codebase is mature and stable for practically everything except media capabilities, where work stalled years ago and has never resumed. You may find that some media is badly encoded and re-ripping from the original sources or re-encoding makes things more playable.
NB: mkv is a container not a format so it's hard to comment further without knowing what the actual format was (although HEVC is the most likely answer).