I was able to reproduce the issue you described, and found an existing bug report related to this. The issue has not been resolved, but you can follow the status of this at the link below:
For the time being, you should be able to simply restart the engine after placing and sizing the Nav Modifier. Once you reload your project, just move the Nav Modifier slightly and it should draw correctly.
Please let us know if you have any further issues.
Currently, our development resources are dedicated elsewhere and UE-30174 is not a priority item. We don’t have a timeframe for when this may be addressed. Please keep in mind that with source code access, a programmer on your project can implement your own solution, and you are welcome to share that result with Epic for possible integration.