Why do I get such a response "Provisional headers are shown" (Angular 4)?

Question

JavaScript

Need help. A more detailed description of the situation:
when choosing a value from select, two requests are sent in asynchronous mode, on the 1st I always get a normal response (respectively, the data), and on the 2nd such a message "Provisional headers are shown", while the response is empty . Such a situation in chrome, but in firefox and explore - the 2nd request is not shown at all in the developer's pencil case. It seems that the request does not reach the server for some reason.
What tell me?
https://hsto.org/webt/fv/jk/vq/fvjkvqrl_tken1dt1_n...