About glossy and reflexion

Why the reflection is so bad ???

Its SSR.

BTW That kind of question you should ask on BA, not there.

Yes it’s a bug SSR but how to fix it ? and what is BA ?

SSR means Screen space Reflection - You have panel with settings enabled on right hand side of screenshot.
No, it is not a bug. Its just how this effect work.

It’s ‘cheap’ screen space effect, You have same problems in UE4/Unity.

BA is blenderArtist forum, i’ve mendtioned it cause it is know/how problem and not a real bug.