Seems to be a widespread bug.I'm seeing the same issue as OP. The Mail app pops up regardless if it's "hidden" (i.e. I hit the red button to close it out of the window) or if it's visible in its non-fullscreen form on the desktop. It pops into full screen and takes up the right side split view when this happens, very frustrating. I've seen this happen on older updates before but they were incredibly rare; after updating to 10.14.5 this happens about every 10-15 minutes.