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.
Ah, I suppose I did word that poorly—I more mean that a significant breaking change (Casks that previously were documented as being excluded from auto-updating suddenly being auto-updated) which can occur silently is a rough end-user experience, even if the user explicitly opted into hiding hints.
I read this as "This means if you close your eyes you don’t see things, which is a bummer."