SERVER-49866 added an invariant that if we get BackgroundOperationInProgressForNamespace applying an oplog entry, the node must be in initial sync. After SERVER-59399 this is true since we no longer assert that there are no index builds in progress on a collection in order to drop a different index on that collection. However, if the FCV is not upgraded, we actually still throw this assertion. Thus, if the FCV is not fully upgraded, attempting to apply a dropIndexes oplog entry in steady state replication can result in BackgroundOperationInProgressForNamespace if there is an active index build in progress on that collection.