This gets curiouser and curiouser. After several days of not seeing fare classes on the app, even after turning off expert mode, then saving that setting, then turning it back on and saving again, I tried something new. I did my usual search for my desired flight, but first turned on "search award travel." No fare classes were displayed, nor was I expecting they would be. I then clicked "Edit" and toggled from "Miles" to "Money." Lo and behold there were the fare classes.

I've now done several searches with "Money" as the form of payment, and the fare class display comes up every time. Go figure. I'll see whether or not this holds for more than a day (or even an hour), but at least I know it can be made to work.

Edited to add: Happy to report that after doing this fix once, expert mode has worked consistently for the past couple of weeks, without the need to use the fix again.