Program just simply replace wallet number, nothing else. As I recall, there were discussions in Claymore Ethereum Miner topic about stale shares might be related with Dev Fee itself (something about it commits share to dev fee and your wallet at same time, not sure if that is the case though) . Also after Byzantium it seems like stale share increased, not sure if that is related either.

I dont know about other pools or why dev fee switching pool (never happend to me). As always feel free to send me log, I will look what can be fixed.