It’s a suspected Counterparty bug related to the way Polo’s application works (which is different from how others use CP so this problem seems to be affecting only Poloniex).
Recently the developers and Poloniex have narrowed it down to one specific area and a fix is being worked on (technical details can be found here: https://github.com/CounterpartyXCP/counterparty-lib/issues/875).
Hopefully by July this will be fixed.