Hello,
I have noticed that auto-completion works well for commands, i.e. it inserts the correct command and locates the cursor inside the curly braces, if any. For environments though, the completion is done in addition to the text already typed.
For example, typing
This autocomplete function works really good, but you have to use it a bit different. For this particular environment (itemize) you have to write only "item" for example and press CTRL + SPACE, and you'll get: