Thanks for reporting this bug, we'll address this in the next release.
From my testing, I found this only affects the sharing dialog when you have a scrollbar in the groups listbox, as there are too many groups to see initially. I also found that when you share with the last group in the list, although the checkbox for the first group in the list initially appears as checked, the item is not really shared with that first group - if you scroll up and down a bit, I find that the first box then gets cleared again (unless I do actually select it).