PD,
Some pictures would really help understand this. Can you post any?
I'm concerned because I use and sell Tonkin Bamboo shafts (Mahill Archery on Ebay) and might be able to learn what to look out for myself and warn my customers against. The question is whether this is conncected with any defect in the shafts, and whether it could be prevented by a different method of arrow construction or finishing.
Have you looked closely at your other arrows to see if there is any sign of splits in the fletching area? Any indication where in the arrow the split started? This can often show where the problem is, like when a bow lifts a splinter or break pointing to where it was stressed.
I wonder if this may have anything to do with moisture loss? A bamboo pole will split if it dries quickly so I guess a smaller arrow shaft might do the same if it got wet, took on water and then dried quickly again. What kind of waterproofing has been applied to them and are you in a particulalry dry area?
Also, do you know what kind of bamboo these are? Tonkin or something else?
Mark in England