I have a very simple journey with a Data Extension entry source with around 290,000 records, each corresponding to one contact. The Data Extension has a correct Subscriber Key relationship (Salesforce originating PersonContactId mapped to Subscriber Key). It has no filter limiting entry, no decision splits, is set to re-entry anytime and was scheduled to run only once. It uses the Email from Entry Source as the Email for the send.
Out of 290,000 contacts, only 141,500 Contacts have been evaluated for entry. What could be the reason? A few contacts out of these could be SFMC unsubscribed but definitely not the whole missing 150k.