Music Duration column doesn't show anything for the .aac files (Explorer's Duration column doesn't either, but then again Explorer's Duration column doesn't do anything for the .mp3 files either but Opus does).
If I wrap it into an mp4 container (using ffmpeg) then Opus does show the duration.
I don't think there is a fast and accurate way to get the duration of a raw AAC file. You have to parse the entire file, which would be very slow for a column that has to quickly get data about every file in a directory. (Or you can estimate it based on file size and a guess of the average bitrate by looking at the start of the file, but that won't be accurate.)
https://hydrogenaud.io/index.php?topic=118687.0
Just one of several reasons that AAC should usually be put inside a container like MP4.
I see, fair enough! 