I am not sure if this is a bug. But I found, if the folder has a “.” Within its name. In the extension column it will show the letters after the “.” As extension.
Screen shot
The file name is “Possession.(1981).720p.BluRay.FLAC1.0.x264-ShitBusters”
Directory Opus is considering the folder extension as .x264-ShitBusters
You may have turned on Preferences / Miscellaneous / Advanced [Information Display]: display_folder_extensions, which would cause what you're seeing in the first post.
(But if that's on, presumably you wanted it that way at some point? That said, the docs on it were a bit vague, so maybe you thought it did something else. I've clarified the docs a little for the next update.)