When you upload an XDP template through Forms&Documents UI, it carries out a couple of processes in the background. This includes creating a DAM Asset structure for the form, adding an identifier for the form type (XDP) and extracting metadata from the form content bytes. So, in short a form in AEM Forms is not simply bytes stored in the repository but an asset with a defined structure and certain additional information stored along with the bytes.
So, when you upload the XDP file via webdav, I guess it would not be hitting our REST endpoint and so the processes will not run. The final structure of the XDP file would not be the same as that expected by the AEM Forms UI. We do not really support uploading form bytes from an external source outside AEM Forms&Documents UI.
Is there any specific reason you wish to upload the form via webdav?