Problem in a workflow in which I'm computing the dynamic approver

I’m writing a workflow that is able to calculate an approver dynamically


as you can see the workflow is retrieving some information of the requested item trough the first HTTP request and based on the results it is going to a loop or to the HTTP request 4. The point is that if the flow is going to the loop, it works correctly, while if it is going to the HTTP request 4 , I have an error of “invalid URL format”. The strange thing is that the HTTP request 4 is equal in the parameters to the other HTTP requests that are working:
HTTP request 4 parameters:

Request URL: $.trigger._metadata.callbackURL
Method: POST
Request Content Type: JSON
Request Body: {"output":{"id":"4a2e2107-2955-479c-bda3-b5062633a400","name":"FCP_Approvers","type":"GOVERNANCE_GROUP"},"secret":"{{$.trigger._metadata.secret}}"}

HTTP request 2 parameters:

Request URL: $.loop.context.trigger._metadata.callbackURL
Method: POST
Request Content Type: JSON
Request Body: {"output":{"id":"4a2e2107-2955-479c-bda3-b5062633a400","name":"FCP_Approvers","type":"GOVERNANCE_GROUP"},"secret":"{{$.loop.context.trigger._metadata.secret}}"}

How it is possible that the same request is giving me in the HTTP request 4 the problem of invalid URL format? Is the loop that is like stealing the variable of the external trigger and I can’t see it in the HTTP request 4?