Volume Structures Test Fails If APFS Container Has Two Volumes

On two different external APFS drives, Volume Structures runs normally when there is only one volume. After a second APFS volume is added to the container using Disk Utility, Volume Structures stalls on each volume with the selected volume unmounted and the Freeze Volume light blinking. If the added volume is removed, the test again runs normally on the remaining volume.

My system: 2018 Mac mini, Mojave 10.14.6, TechTool Pro 11.0.5. One test drive was a USB spinning disk attached directly to the Mac, the other a Thunderbolt SSD attached through an Akitio Thunderdock and Apple Thunderbolt 2 to 3 adapter.