What is actually happening is that when you choose DTO terrain in the Terrain dropdown, two other terrain transformations are being applied. If you pick DTO Transformations in the transformations dropdown (next to the buttons), you will see that DTO Broken Terrain, DTO Steppe Terrain, and Use Desert Colors for ETO boards are all checked. You can leave DTO terrain selected in the Terrain dropdown if you like, but uncheck DTO Broken Terrain and DTO Steppe Terrain (unless, of course, one or both apply). If you have no ETO boards in your scenario, there isn't really a need to perform any transformations. I have filed a vasl issue:
https://github.com/vasl-developers/vasl/issues/520.
JR