Filtering and modifying simultaneously DbSet - entity-framework

Question

I'm working on the DbSet where I'm modifying it and filtering data I have modified before saving those changes to database.
Initial content of table:
Id,Name,Left,Right
1,All,1,28
2,Press,2,13
3,NewDeals,3,4
4,Contracts,5,6
5,People,7,12
6,Promotions,8,9
7,Deaths,10,11,
8,Finanse,14,19
9,Results,15,16,
10,Stocks,17,18
11,Stories,20,25
12,SuccessStories,21,22,
13,LearnStories,23,24
14,AccessToEverything,26,27
Steps looks like this:
//items are DbSet
var nodeLeft = 10;
var nodeRight = 11;
var parentNodeRight = 28;
var size = 2;
var itemsToUpdate = items.Where(i => i.Left >= nodeLeft && i.Right <= nodeRight);
foreach (var item in itemsToUpdate)
{
item.Left = 0 - item.Left;
item.Right = 0 - item.Right;
}
// step 2
itemsToUpdate = items.Where(i => i.Left > nodeRight);
foreach (var item in itemsToUpdate)
{
item.Left = item.Left - size;
}
itemsToUpdate = items.Where(i => i.Right > nodeRight);
foreach (var item in itemsToUpdate)
{
item.Right = item.Right - size;
}
//step 3
itemsToUpdate =
items.Where(i => i.Left >= (parentNodeRight > nodeRight ? parentNodeRight - size : parentNodeRight));
foreach (var item in itemsToUpdate)
{
item.Left = item.Left + size;
}
At step 3, where statement is returning values which does not fulfill the statement in where.
It looks like EF is filtering the data which are not in memory but in DB.
If I change items to regular list by applying toList() everything is working fine.
I would really like to avoid that since data set I'm working on can be very big and I can not initially pre-filter it.
Could you please advise what I'm missing / doing wrong here ?