We have the "windowTarget": "_blank" working for LTI 1.3 tools, but don't have a specified on the developer key, but on the placement in the developer key, I don't know if this is the problem you are seeing but might be worth trying.
I assume you're using the AGS service (https://canvas.instructure.com/doc/api/line_items.html) to create the line item? It doesn't look like it's set in the assignments too either from your other screenshot, are you sure your request is working? If you try to set the max score to something other than 100 does that work?
If you're wanting a new window because you don't have HTTPS in development, I'd recommend having a look at something like https://github.com/FiloSottile/mkcert to setup HTTPS in development as browsers also limit some features to not work on HTTP.