What is the point of trying to read paths from the frontmatter and what is the expected syntax from Meta Bind plugin? What is the ! doing there?
I don’t know. I am asking you. Did this really use to work?
Why not add template file and folder as they are expected and be done with it?
Or, alternatively…does a template populate the paths beforehand?
Then maybe explore ways to add the Meta Bind button through that earlier template, in which you can dynamically use variables such as <% `${variable}` %>.
I am not a Meta Bind expert. I don’t go to sleep and wake up with it.
From what I’ve encountered, there are two main use cases: create buttons on a dashboard or query page or dynamically through templates.
For example, in the SoRobby demo vault, the author uses Buttons plugin buttons in this template of his.
I converted the method to Meta Bind months ago, but didn’t even end up using the template based on the linked one.
So what I’m really asking…are you not overcomplicating something here and confusing us with it as well? It’s quite common on this forum, from what I’ve seen… 
That is why the exact workflow is needed in the posts, but of course, if the post is too long, potential helpers may not have the time to read through it…