Spits can supposedly hold 15KN on a straight pull, and the petzl resin anchors can withstand 15KN on a straight pull, if installed correctly, therefore P bolts would be comparable to this 15KN figure. Therefore they should easyily shrug off 6KN as this is less than half the rating. I believe it is more likely to be something environmental to the region, either the more fractured limstone, which has already been mentioned or something within the surrounding environment of the anchor such as the surrounding air or water reacting with the resin, either once in place or during the curing process. I think the later more likely.
The bolt that failed initially looked as if the resin had not bonded propperly to the bolt as the seperation between the two was very clean, no resin remained on the bolt and a perfect mould was left in the resin. Leading me to think along the lines of water ingress along the bolt during curing preventing a good bond between the two surfaces. I dont know the exact chemisty of the resin so this might not be an issue.
Alternativly disolved salts in the rock, higher than average acid concentration in the water, or any combination of things could react with the resin while its setting, weakening it or preventing a good bond between surfaces. Again i don't know the chemistry that could be behind this and I am just theorising.
I'm guessing as no one has said anything the re-opening of the cave is still some way off