Tree sort order changed after refresh

Current Situation
Initially the tree sort order is from 'a' to 'z' case insensitive.
After an update of the tree through an event the sort order changed to 'A' to 'Z' and from 'a' to 'z' behind the items starting with upper cases.

The sort order is now case sensiitve.

Desired Behavior
The sort order of the tree should not change after an update and should stay case insensitive.