I have no idea whether losing back tension would cause string slap. I do know that loss of back tension is destructive to the shot process, to a much greater degree than string slap. If you solve your loss of back tension problem, your string slap may go away. If it doesn't, you're left with a minor problem rather than a major one. I don't think it has anything to do with dropping from 55 to 50 pounds, or changing from the Bob Lee to the Bear (unless something is wrong with the Bear). My experience is that the grips on the Bob Lee and the Bear are very similar, so I doubt if that is causing the problem.