You need to set HOMEBREW_NO_UPGRADE_AUTO_UPDATES_CASKS to 1, as alluded to by a hint when it (first?) occurs. This means if you have hints off (via HOMEBREW_NO_ENV_HINTS) then I suspect you can start getting this behavior without warning which is a bummer.
See also: https://docs.brew.sh/FAQ#why-arent-some-apps-included-during...