From linked CBSE we see this warning many times.
This code is driven by the expiry pager, it is trying to load the value of a now expired item which is tagged as "xattr":
However "seqno:-4" indicates this is really a state_non_existent_key StoredValue, which occurs due to bgfetch. Each time getValue fails to just find the document, because it genuinely doesn't exist we still increment failure_get
This is a side-effect of the bug described in
MB-47952 - having deleted, non-resident documents in memory which (incorrectly) have the XATTR datatype set. As such we attempt to read a document which no longer exists, and hence the above error is seen (and failure stat incremented).