I think that creating a new deployment when one already exists can lead to problems.
But I'm not sure if that is your current problem.
When I publish a new add-on version, I use the legacy editor, and choose:
File -> Manage Versions
Add a new description and click "Save new version"
Also, in the App Configuration for the "Forms Add-on Project Script ID"
I enter the "Project key" NOT the Script ID.
There was/is a bug I believe when trying to install from the marketplace listing when you go directly to the link in a browser.
If you open up the Marketplace listings from inside the Doc, then I believe it works.
But I don't know whether your current issue is related to that bug.
So, what to try now? Can you choose "Manage deployments" and get back to the deployment for your version 6?
I'm wondering if there is a way to edit the deployment that was associated with your version 6.
But I'm just guessing, I don't know if it's a good or viable idea.