Long description, sorry.
But it is annoying since.. I can't even remember when.

TL/DR:
1. Open template with items
2. Create new trigger
3. "Add" into expression field
4. "Select" item
Expected result:
List of items of current template
Actual result:
List of items of first (alphabetical) group and first host from that group.

So, here is the thing.

When we are creating new trigger in a template, we need to add some items with functions into the expression field.
We have the button "Add", we got new dialog window with another "Select" button which helps us to choose item we want to use.
We click on "Select", and ANOTHER new dialog window opens.
BUT!!!
It opens with first group from all of the groups, and first host from that group.
And we need to change group to "Templates" and host to current template we are editing right now EVERY TIME.