Assume they already know about all the bugs you found.
Their app is in Beta. They have QA testers. They know about the bugs, and probably more that you haven't found. They shouldn't be using candidate interviews as a means of testing their own app, and it's highly likely they are not.
So if you do decide to bring up a bug, do so with this in mind. Remember that you are telling them what they already know, and they are definitely much more familiar with their app than you are. The point of telling them becomes a demonstration of your own ability to notice detail, and you can do this by pointing out holes in other apps too, ones unrelated to theirs. I don't suggest you bringing up a bug unless they ask for it.
If you'd like to tell them about the bugs you found, use the bug logging system they suggest (usually with a Beta, they'll post advice on what to do if you find a bug. Follow that).