iTunes 12.9 won't allow change to media type on multiple files at a time

I'm resurfacing an issue that I found some information on for prior builds of iTunes, although this problem just started showing up for me on version 12.9.5.5. The old 2018 fix I found didn't work. I am still running Mojave, FYI (10.14.6) because of some 32-bit software I use.

Audiobook MP3 and wav files have to be imported into iTunes manually, as music, and it will no longer allow me to change the media type to audiobook on multiple files at the same time when I want to put CD books from the library onto my iPod or iPhone, forcing it to be done one file at a time. Since CD (.wav) & MP3 audiobooks are often made up of as many as 200 files, it can be a crazy time-consuming task.

It used to works like this: after importing an audiobook, the next step is to change the media type to "Audiobook" under the Options tab. So you highlight all of the files in the Song view list, right-click on "Get Info," and go to the Options tab, but the only choice offered is Music. But if I select the files one at a time (by right-clicking and selecting "Song Info"), then it will offer me other Options on the "Media Kind" drop-down (including Podcast, Audiobook and Voice Memo). But not if I select more than one file to change at a time. If it's more than one file, the only option offered for Media Kind is Music.

Any help or suggestions would be hugely appreciated.

Thanks.