While replacing my calipers and their mounting brackets, I passed a tap through the bolt holes on the wheel hub assembly that secure the bracket, to clean up the threads. The tap's movement felt unusual: it didn't seem to be hitting any thread once past a certain point in the hole (which is about 3/4" deep in total).
I took a closer look, and found that there doesn't appear to be any thread in approximately the second half of the bolt hole. There might be the remains of some; it's hard to tell:
The bolt starts on the inside of the car; the picture is taken from the outside -- the back of the hole where the thread seems to be missing.
The bolt seems to hold its position just fine. I had the bracket off a few months ago, as well as sometime last year, and I'm always careful to torque things down to their specifications. I didn't check the torque when pulling it off, of course, but it required force to loosen that was definitely in the right ballpark.
Both holes have the same appearance, which makes me think it's more unlikely that this is abnormal. The other wheel is about the same; there might be a little more thread. Hard to tell. Is this how the threads are supposed to be, or do I need to plan on replacing the hub assembly?
(I can't see enlargement and retapping of the hole as a solution, because I don't think there's enough surrounding material on the bracket itself.)