Negative HTTP response phrase always OK after ugrade for IIS 10

My local developer environment is IIS 10 (comes with Windows 10 update) using PHP FastCGI. I am working on some API endpoints and all PHP headers such as header(HTTP/1.1 400 Bad Request);

and responses header('HTTP/1.1 405 Method Not Allowed');

include the phrase "OK". 200 OK, 400 OK, 403 OK, 404 OK.

This works great using the same codebase on machines running before IIS 10. Any ideas on what to look for in IIS configuration, FastCGI settings, etc. to fix this issue?