As explained here there was a pretty significant flaw introduced way back during Linux kernel 4.19 development having to do with page caching. The most obvious effect with Unraid was to cause spin-up of drives not involved in large transfers due to cached device top-level directory i-nodes getting reallocated prematurely. The latest kernel patch release (4.19.24) fixes this issue.
More webGUI refinements are also included.
Version 6.7.0-rc5 2019-02-22