According to a Qora block explorer, on a node at height 452384, Poloniex's last transaction was on 9th April 2018 at block 446780:

http://node.qora.tech:9090/index/blockexplorer.html?tx=2bYmw35XQfPu6kcVq9kEphbto3uTprwzrdYSti6yPq3eHqUAJcpCCy5arAMUC3M9hMA8Q39zCt2atxFaFHxgYg9N

I don't know which version they're using but they should really upgrade to the latest v0.26.9 which deals with recent synchronization issues.
Maybe they are sending out transactions only for them to expire as they don't make it onto the majority chain.

Newer versions of Qora synchronize way faster so they could even do a fresh install just to be sure. I hope they have a backup of their wallet seed though!
There are still people out there using v0.26.0 which does suffer from database issues/crashing that causes resynchronization...