Connection string doesn't work as stored Variable, but does in JSON body

I’m trying to create a new set of cognitive search tools in azure via postman, and following the microsoft tutorial. I’ve created all my variables, and attached the relevant connection strings. When I try to use the POST command to create a new data source, I get the following error:

Changing the body JSON to call it without using a variable seems to work just fine:

It’s the exact same connection string, copied from the variables. I’ve tried using every connection string variant I could find from those listed in the error message, but they all give me the same error.

So the question is, have I found a weird bug? Or is this a user error?

What is it showing in the console log?

What was actually sent? Is it perhaps taking it as a literal string, and not reading the variable?