Just tried your code on a sample and it works fine for me. If it works with a new form then there must be something unique with your form. Can you post it to Livecycle8@gmail.com and I will have a look.
I noticed a couple of things. The sample which works for me is very
simple, no data source and the default binding for the list box is set to Normal. If I change the binding to None, it stops working. Change
it back to Normal and it works again.
I think it might be the version of Reader that I'm using, which is
8.1.2. I found this note in the 8.1.3 release notes:
"1570212 Multiple selection listbox does not keep items selected when focus leaves field."
I upgraded to 8.1.3 and it seems to be working as expected now.