Disk Not Ejected Properly Notifications

IMac running 10.13.6.

I have an external 64 Gb USB 3 flash drive that stays connected to my iMac all the time. The disk contains a single file, which is an encrypted sparse bundle disk image. The sparse bundle is mounted as well.

Whenever the Mac awakes from sleep, the screen is full of a flood of "Disk Not Ejected Properly" Notifications, and the sparse bundle disk (but not the physical disk on which it resides) has been unmounted.

This started as of the 10.13.6 upgrade.