Yeah that tracks - Good add-on: in low light, use a single center (cross-type) AF point and aim at a high-contrast edge like a collar or eyebrow, and turn on the AF assist lamp or flash assist if you have it. Do a quick AF microadjustment for that lens if your body supports it, and clean the lens/body contacts to rule out hiccups. Back-button focus plus a higher minimum shutter speed in Auto ISO can also cut down on misses with moving subjects.
Cracking after a month? That's rough, especially with that model work throwing all sorts of abuse at it... I bet it's the material not holding up to repeated impacts or maybe even the way it fits the phone, causing stress points which, yeah happens more than you'd think with inexpensive stuff.
I've switched to tougher designs that wrap around better and it made a difference, no more constant worries. If budget's tight, hunt for deals on sites with good return policies. Anyway, hope you find something that sticks around longer this time.
