There is 2 type of bitcoin adresses prefixes .
Originally those  starting  with 1 is Pubkey hash (P2PKH) addresses . This with 3 is Script hash (P2SH address).
P2pool code originaly distribute shares for work  only to P2PKH address, which is  set for mining user .
Similar to MUE P2PKH address start with 7 and is accepted and recognized by p2pool code as user and recive shares for work. P2SH address is accepted on any p2pool mined coin as user , but don't recive shares for work and all shares go to default p2pool addy , such as  wrong addy with missing sign or address from different coin.
MUE P2PKH addy starts with 7
MUE P2SH addy start with 4 or 5
 P2SH addresses are generated usually with web wallets .