How to make audio file playable as a podcast on iPhone

The file extension is .mp3. Some of these same files previously had no problem syncing to the Podcasts app, and although they're entirely unchanged, no longer do.

I do have 11.0.2.26. This problem has existed at least since version 11.0 debuted. Unfortunately I don't recall if it started when I transitioned to 11.0 or before that, but I know this has been ongoing since at least then.

I've tried switching the genre to podcast, but this didn't do anything (I've left it as podcast anyway).

I also tried another mp3 and it synced to Podcasts correctly. I took a closer look at the details of the file types and the ones that aren't working are MPEG-2 Layer 2. Could this help to explain the issue? (Although I still don't understand why it worked with them before and not now.)

Something else odd happened in this 'test'. After syncing it to the Podcasts app, I then deleted the file on my phone. I checked the Music app as well to make sure it was gone (it was). I then checked with Spotlight search, and for some reason it showed up, although the icon next to it had the design for the Podcasts app yet was colored orange like the Music app. I clicked it and it opened in the Music app and in the Now Playing screen a new small button appeared under the seek bar, where the shuffle button usually is. The shuffle icon had moved and instead there was an icon that controlled the speed of playback like in the Podcasts app (though the only options were 1/2x, 1x, 1.5x, and 2x--no 3x). I tried playing another song and this icon disappeared, but when I went back to the former song, it reappeared. I've never seen that happen before...