I kick myself for caving in and upgrading wish I had remained on 10.14.3 for a while longer. Beach Ball, Safari freezes, Time Machine weirdness. I've noticed several oddities after upgrading to 10.14.4. Beyond, the usual power off/on, reset this and that, unplug, recable suggestions, no help. I was in chat with Apple Support (twice) today. The 1st volume is a CCC backup drive the 2nd volume is a TM backup drive.
I have no idea what that means, if anything.
The only difference I see between my two external USB drives is that the 1st (auto mounted) volume carries a TYPE of USB External Physical Volume while the 2nd (no longer auto mounted) carries a TYPE of CoreStorage Logical Volume. After mounting the volume, the system and apps once again behave (somewhat*) normally. It has the same attributes as you indicated - Mac OS Ext Journl/Encrypt. Like you, I discovered in Disk Utility that the volume was not (auto)mounted. Both drives are connected to a TB3 dock which in turn, is connected to TB3 port on the Mac mini. And no issues with my other external USB drive. Control which programs start when you launch Windows. In my case, a second external USB drive is 'gone' from the desktop, gone from Time Machine, etc. Download the latest version of Autorun Organizer for Windows. I have the same problem now, immediately after upgrading from 10.14.3 to 10.14.4 (never happened before the upgrade).