SEO оn and category menu lose subcategory trace

So i have several categories with subcategories, and when SEO is on this happen:

Main category 1
Main category 2
Main category 3

1. Click Main category 1 -so url is site.com/Main category 1

and in the left the category box is expanded and shows the sucategories, also in the content part it show all the categories and bellow them the products in Main category 1

2. But when you click from there any of the subcategories
Main category 1
- subcategory 1
- subcategory 2
- subcategory 3

It shows all the products in the content, but in the left category box the menu is collapse to:
Main category 1
Main category 2
Main category 3

and it shoud be:
Main category 1
- subcategory 1
- subcategory 2 - for example when you've clicked this subcategory
- subcategory 3
Main category 2
Main category 3


All of this works when SEO is off ?

So any ideas, and if not bug what may cause the problem ?