macOS public beta stopped updating
I'm unsure if this solved my problem, and I'd welcome a better answer, but after asking, I


Toggled beta updates off
Rebooted
Re-enrolled my Mac; no updates found
Ran softwareupdate -l; but no updates were found
Ran softwareupdate --dump-state to try to debug
While I was rooting around in the dumped state and downloading and inspecting files, Public Beta 5 became available and showed up both in App Store.app and softwareupdate -l


I'm guessing that the toggling off and re-enrolling of beta updates made me eligible for the next Public Beta; though I'm not sure. I'm posting because perhaps that strategy may help someone else.