Would this be with newsgroup accounts, where the body text is only downloaded on demand and thus isn't searchable?

One way to search newsgroup messages is to work with Thunderbird offline, which obliges it to download message bodies en masse.

Thanks for getting back to me, Zenos! No, it's not with newsgroup accounts---only with just plain vanilla email. I segregate email into sundry folders (e.g., Financial, Friends, Volunteering and the like), and when I open one of those folders and then do a Ctrl+Shift+f, that's when sometimes I find "Body" in the Subject dropdown, and sometimes I don't. I hope that's clear.....

Hmm, yes after a bit of experimenting I see it occasionally goes missing, but I can't work out the pattern. Spam, Bin and Trash seem to be locations where it isn't offered.

And interestingly, if you start a search, switch to a different folder which does offer Body, add the rule, and then switch it back to Spam, the Body rule greys out, clearly becoming disabled. So particular folders do seem to be specifically and intentionally excluded.

Again, thanks, Zenos, for your interest. Once when this happened, I re-started Tbird, thinking a bit got flipped somewhere. That happened to work; the next time, I actually had to reboot the computer. That happened to work. But with today's incident, neither of those worked. And this artifact is occurring in folders I've successfully searched (yes, using Body) dozens of times in the past. Very mystifying......maybe someone else will have experienced that and determined its cause; we'll see!