Pleased to meet you - I'm Kano.
Yeah it only affects the BestShare value - it doesn't actually mean you lose shares.

However ... the fact that their code can throw away valid blocks on p2pool is the really unbelievable one

95% of stale p2pool sharechain blocks are valid blockchain blocks - so your local p2pool submits them to the bitcoin network anyway - coz yeah that would be bad not to ... except ... the bitmain version of their driver discard stale work before it gets back to the work processor in cgminer
I did of course fix that back with the S1 (and S2) ... but ... they ignored all the changes I did then or in the S2 ... c'est la vie