Bitmain does some terrible things in their driver including ... discarding stale work in the driver before passing it to the work validation code.
This means on p2pool that if you find a stale block that is still a valid network block, it will be discarded. Oh well.

Use my driver for S1 if you don't want to throw such valid blocks away on p2pool.

My S2 driver is still in development - I'm still trying to work out why the device thinks it's a good idea to have >8000 work items queued in it - more than 30s of work - and to work out how to reduce that without letting it get idle - so I'm still waiting on source code from bitmain.