Inbox placement rate (IPR) is distinct from delivery rate: a message can be "delivered" in the sense that the receiving server accepted it, while still landing in a spam or promotions folder rather than the primary inbox the recipient checks.
IPR is typically estimated through seed-list panels rather than measured directly, since senders don't get authoritative folder-placement data from most mailbox providers for real recipient sends.
Falling IPR with a stable delivery rate usually points to a reputation or engagement problem rather than an authentication failure, since authentication failures more often show up as outright rejection instead of folder placement.