I dont really think this is a bug but more a quirk, but since it works right in some places but not others, im gonna assume its something not exactly intended.
When typing in a Structure name and its Child Variables in a Variable field, it auto completes just fine.
As your typing it starts giving you the options that you might want to help you finish writing, and if you select it, it will just place it in the field and and the auto complete goes away, perfectly useful and awesome to have.
If you try to do the same thing in other values, like for example an Objects X and Y Position, it dosent do that.
It still gives you the auto complete options, but it gives you all of them and no matter how much of the variable you type in, it never updates.
If you use the keyboard to select the option and then press Enter or use the mouse the clique the one you want, the auto complete value will go onto the Field, but the auto complete options wont disappear, making you have to clique off the selection so you can move on. Which is rather annoying depending on your patience at the time 
This has been around for as long as i remember, but i supose it finally annoyed me enough times for me to write about it 