When you apply a filter descriptor, all entries that don't pass the criteria will be hidden from the UI. In such cases, if the selected item is among those entries, the SelectedItem property will be cleared (set to null). This is an implementation detail of the control - it doesn't make much sense to leave the item in the selection as it's a primarily UI functionality. That's the behavior I'm observing locally and I couldn't replicate a setup in which the binding itself is set to null.
Is this behavior preventing the proper logic of your application? If that's the case, please elaborate a bit on what you're trying to achieve and I'll gladly help with an alternative solution.
As to the difference between SelectedItem and SelectedItems - those relate to the different types of selection modes in the control. You can take a look at the Selection article
for additional details on the specifics.
I hope this helps.
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