Stay with Plex TV Series scanner and Plex Series agent.
Other than audio/subtitle track names and some flags, Plex Media Server does not read embedded metadata from MKV containers.
It will read the info from MP4 containers if you enable Prefer Local Metadata.
FWIW: Consider avoiding embedded posters in MP4 containers. They’ve been unreliable for me, with Plex sometimes failing to read the poster and displaying nothing (all black). I quit using them quite some time ago in favor of external posters/artwork.
That should work if Use Local Assets is enabled.
If you’ve already manually picked a poster, then Plex will not choose Season01.png if you add it later (doesn’t on my server).
You may need to Plex Dance the season or show.
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