Hey @kkukshtel!
Thanks for your message and welcome to the forums! 
Notice that the div with background image has a class applied to it. If you edit the other instances of a symbol by double-clicking it, select the same div in a symbol and see how it has the same class applied. So what’s happening, is that when you change a style of a node outside of a symbol, you’re still changing a style that is applied on a class. And that class is applied on other elements like your symbol inner nodes.
Now… When you unlink a symbol, you can click on the arrow over the class in Selector on the Style Panel and choose Duplicate Class. That way you’ll create a new class with all styles that were on an original one, but you will not affect those in a symbol.
Let me know if that helps
Cheers,
Bart