No Digest Mails - (at least) one user has none

I have a user who is basic member, is not disabled, has allowed to receive digest mails on absence, has configured an email and shows activity back in February.

I have a higher setting for “suppress digest after 600 days” so this cant be the reason the user never got a digest mail dispatched (according to the Admin / Email / Sent list). Are there any other conditions which could inhibit digest creation for them? How can I check?

I see quite a large number of sent digest mails daily but there is no good way to see which users had received any - or which user are missing from that?