I've found the issue causing this.

// This is the `err` I wish to log
if err := validate.Validate.Struct(payload); err != nil {
        // err becomes nil here
        errResponse, err := utils.GenerateErrorMessages(err)
        if err != nil {
            utils.WriteJSON(w, http.StatusInternalServerError, err)
            return
        }

        errorMessage := response.ErrorResponse{
            Message: "Invalid request body",
            Errors:  errResponse,
        }
        // nil err is passed to response writer and not logged.
        response.BadRequestErrorResponse(w, r, err, errorMessage)
        return
    }

I just need to change the use different variable names for the error.