First, read Cal Henderson's excellent "Understanding Bidirectional (BIDI) Text in Unicode.".
Believe it or not, UILabel is laying it out the way you asked. You've provided a right-to-left string (starting in Arabic). It starts displaying that right to left. You then embedded a left-to-right English string, which it lays out left to right. It then sees the period, and lays that out right-to-left since this is an Arabic string that just happens to have some English in it (as best UILabel can tell).
What you meant to have is a left-to-right string with Arabic embedded. That means you have to start the string with left-to-right characters. Two options: add some English to the start, or use the zero-width Left-to-Right Mark (U+200E, LRM) to anchor the beginning of the string into LTR mode.
Objective-C:
self.label.text = @"u200e??? just bought: Disguise Kit.";
Swift:
self.label.text = "u{200E}??? just bought: Disguise Kit."
The good news about U+200E is that you can safely add it to every LTR string before you display it. You can even safely put it at the start of your localized strings for LTR languages. If it's redundant, it doesn't hurt anything.
A couple of other things to note: never test this stuff with ???, always test with ??? (like every good student :D) or better yet something like ???. Otherwise you can't tell when the Arabic is laid out backwards. I like ??? because it looks wildly different backwards.
Also, when testing, note that Xcode doesn't know how to layout Arabic. So if you write any static strings in your code, they'll be displayed backwards in the editor, but they'll display correctly in the UI. Drives me crazy.
Be sure to also see guru_meditator's answer for helpful related features added in iOS 10+/macOS 10.12.
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