There are a couple of threads mentioning this problem. I have a relatively new bucket that does not have any little gussets on the bottom at the edge, but does have a small crack right where the bottom joins the lip down from the edge. Fortunately, this bucket has been used to hold other things less precious than my beer. Several of my older buckets, not only fermenting buckets, have the gussets spaced around the edge and have never cracked.
I get the feeling that a cost reduction in the making of the bucket molds has led to this problem by eliminating the gussets. Of course a lot of buckets are designed for one use applications, not meant for re-use over time.