There's a hidden "forms" folder in each SharePoint document library. When you set a new template (via the modern experience) you can see the template being placed into this folder. I've updated the template via Windows Explorer (because it's still exposed via WebDav) and it works. I have also updated templates via Powershell - so I am assuming a simple file copy operation should do the trick. But no luck.
Is it possible to upload or set new templates in this folder via Flow? I've tried using the SharePoint File Copy operation, and the following is what I get:
{
"status": 400,
"message": "Failed to verify the existence of destination location at 'https://mytenant.sharepoint.com/MySite/TargetLibrary/Forms/' due to error 'The system cannot find the file specified. (Exception from HRESULT: 0x80070002)'.
If I use the https address (in above error message) in a browser, I can drag and drop DOCX files into the forms folder and they do appear in the Document Library's "New" menu as a template.
I have also tried the https address with and without the trailing "/":
'https://mytenant.sharepoint.com/MySite/TargetLibrary/Forms/'