DD+7.1 ch is the track description from within your file (track metadata). Plex has been displaying those for some time – though not yet on all clients.
[from the related feature suggestion]
Basically the grey part is just a string from the track metadata itself while the header (English (EAC3 5.1)) is the actual technical details (language + codec/profile). So from the looks of it you got an English language 5.1 EAC3 track that’s been labelled as “DD+7.1 ch”.
If you want to be super sure you can check out the file using MediaInfo (or check out the Get Info overview from within Plex).
https://support.plex.tv/articles/201998867-investigate-media-information-and-formats/
