Sorry for the delay in getting back to you.
But, those notifications didn’t go out the previous day when we had all the exact same settings (and had new entries in several forums) — except for turning on the “Notify all members of all groups that have access to that forum” option.
That option is considered a new, independent, rule, in addition to the overrides and the roles.
And that option (notify all…) is only for instant notifications and doesn’t affect the digests?
Every instant notification affects the Digests. Without them, there’s nothing for the digests to enqueue for, well, digesting :).
Is there a log we can dig into to find out who got what notifications?
There’s a couple of handy plugins that log mailouts. This one is pretty good: