If an IP-256 frame is going to have an issue, that's where I would expect it to be. The "Y" wishbone shape at the seat tube must flex quite a bit in that area. I've read about a few other IP-256 problems on MTBR and that seems to be the area that is the problem. Perhaps the frame mfr should make that area stronger given the loading, particularly if the rider weight is 200 lbs or more.
As a Mechanical Engineer who has performed many structural FEA's on injection molded plastic parts, this doesn't surprise me. With handmade carbon fiber, the manufacturing process appears to have a lot of variation in terms of structural integrity. Most frames probably turn out fine, but there seems to be a few that don't have as much strength in that area.