I remux into ogg which still supports Opus but seemed to have wider player support. Great way to get tracks that don't seem to be available anywhere other than streaming.
Hmm, maybe whatever I was running into problems with just doesn't recognize the extension then. Sadly I don't remember what it was. Coulda been Plex, which wouldn't surprise me.
A lazy indexer could skip just based on filename, and a well implemented and current (although... Opus is a decade old now?) player would properly detect the Opus stream inside the .ogg file and decode properly.
https://en.wikipedia.org/wiki/Matroska