Tenant Level Content Types - Error when assigning Document Template URL

Hi All! 

Thanks in advance for any help. 

Scenario:

1. Logged in as Global Admin 
2. Created a Document Library in a Communication Site to host the "original" Content Type Templates
3. Uploaded a document and got "Path"
4. Created a Content Type in the Tenant Content Type Gallery
5. In advance Settings for the Content Type I choose Use an Existing Template and enter the URL for the document retrieved from de Document Library in a specific Communication Site (non Hub) via the path option.
6. When I try to save I always get the same error: "An unexpected error has occurred. Please try again later."
7. If a try to upload the document it works, but then I can´t link it to managed properties because I can't change the document type in the library... messing up with internals from SharePoint (I know where it is and how to change it, but doesn't sound like a good idea). 

If I do this at site level ... no worries. It works fine. 

So I am assuming it is either a link definition or a permission issue (or it simply doesn't work!)

Seen a lot of video demos to try and debug this, but they always use Upload instead of link. When they use link in the demo is usually at site level. 

When I upload a document it is saved in the _cts folder of the contentTypeHub... but I can't insert properties from the content type columns... because it is stored as a document. 
Doing this per site is not feasible, because I would have to update each of the libraries. 

I managed to create a Document Library inside the contentTypeHub and link to a template there. And it works. Doesn't seem like a good solution because I'm messing with the inner-workings of SharePoint Online, and can be broken by updates. 

Any ideas on how to host tenant level document templates in a document library that is not in the contentTypeHub ? What am I doing wrong?