Hi Will!
Welcome to our community!
It looks like there is somehow an issue with the improper vertical datum. I have a possible workaround that could be a solution. Could you please create a project coordinate system and the vertical datum of your base coordinates and your base coordinates there manually for a point? After that, you can choose that point to serve as your base point, and the app will perform the conversion. Here, you can find our support tip with the necessary steps.