Why is CORS throwing an error?

Question

CORS

On Vue I make a request for the back end of yii2, I registered everything in behaviors.
localhost:8080 , this is the domain from which I am making a request for a back, the back itself is back.ru/some_path
So the link below gives the answer without problems

back.ru/menu?expand=menu_id,status,link,role&per-p...

, and this link back.ru/menu?tree=1 will

give the following error:

Access to fetch at ' back.ru/menu?tree=1 ' from origin ' localhost:8080 ' has been blocked by CORS policy: No 'Access -Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

How would I ever encounter this for the first time, that an already configured cors behaves like this, how does it not give the same domain access to different URLs?