From some quick hunting around, it seems that Bitcoin Core may accept but not broadcast a valid (signed) raw transaction until it is fully synced. I haven't been able to fully confirm this yet, but did observe this myself when trying to broadcast a Bitcoin transaction from two non-synced clients (nb both were synced well past the input transaction).

If this same behaviour is present in the CLAM client, this may explain why real txids are being shown to Poloniex customers, but the transactions don't immediately make it out to the network: the Poloniex CLAM client is not reliably syncing.