I've been building a castle and had some issues with pStones, Nyssa can attest to this. Firstly we thought it was due to fields overlapping. Yes I had '/ps on'! The pStone was actually disabling upon placement, it showed no visible field. So upon breaking it, I of course got lapis dye. Once I moved the stone roughly 5 blocks up, whereas the fields wouldn't overlap, this seemed to fix the "BUG".
Once we "thought" we found a solution I began counting and building up, over 60+ blocks away from ANY pStone, I placed another lapis pStone. Again, no visual, and the stone broke into Lapis dye. This leads me to believe that it's not solely an overlapping issue, but a core issue somewhere in either the WorldGuard code or the pStone code... though I'm only a web developer and don't write Java
I have refrained from placing pStones until I can confirm this has been fixed as I've lost 3 already, which Nyssa was happy to replace being that both her and Keth seen them broken. Anyhow I hope this helps the devs!