The purpose of that bolt on the inner side of the bracket is simply to keep it from sliding forward or backward as the chains pull it when the bars slide forward or backward when making a turn. That is why the instructions usually say to only tighten it down a turn or less after contact. Doing more than that will distort the bracket and make what happened to you more likely.
Yes, many members have bolted the brackets on. This is especially necessary of you have a heavy tongue weight (1200+ lbs). The pull of the chains will sometimes bend the bracket so it fails just as you have described. Bolting the bracket on solves the problem in most cases. Also make sure you have at least 5 links of chain between the snap up brackets and the bar. This allows room for the chains to move without damaging the bracket.
There have been quite a few posts on the subject.
See these for some that were posted by member Turk2500 who bolted his on his trailer. Those should give you some ideas on the cause and the cure.:)
Barney