Stopping a lock when uploading a file tor a Content Type

[metkhoo]

I have found a strange issue, if you have any ideas, I am all ears.
I am trying to programatically upload and set a word document as a template on a new list content type.
I can create the Site content type OK
I can upload a file to SiteAssets (*.docx) OK
I can retrieve the Site content type OK
I can then set the template to the URL of the uploaded file OK
I can add the site content type to a document library OK
I can do this TWO times OK

However

When I create an item in the doucment library with the second content type, it allocatest the FIRST content type to the new document.
If I attempt to CHANGE the content type back to what is was created with I get the error:

Error: The file is locked for shared use by {me}

If I wait around for a few minutes then the lock seems to go and i CAN change the conent type:

Workaround
My current workaround is to go back to the content type directly, use "advanced settings" and then upload the file again to the content type.
When I do this it all works correctly and content type fields are correctly set to the content type being used when creating the item.

I can't seem to find a pnp.Core mechanism to mimic the mechanism behind
"Upload a new document template"
when editing a content type
The only mechanism available seems to be to use the
"Enter the URL of an existing document template"
Which I can't get working

I checked CSOM and it has the same limitations!

Any ideas anyone?
Thanks