Because background validation looks back at the previous checkpoint timestamp, it sees collection documents at that time, however the timeseriesBucketsMayHaveMixedSchemaData flag now lives in the untimestamped WT table app_metadata, and as a result may be viewed further in the future.  If a user alters data in a way that is necessary to make valid a new change to this flag, background validation may for a window of time incorrectly fail this check.  This was the cause of https://jira.mongodb.org/browse/BF-34441 which is for now being resolved by applying an fsync in the failing test.