If you have more than 4 orders per symbol with a gross value less than 0.0025 XBT each, your account will be labelled as a spam account and all proceeding orders that are smaller than 0.0025 XBT in size will automatically become hidden orders. 

 

Spam Account designations are re-evaluated and lifted automatically every 24 hours provided your trading has changed. - https://blog.bitmex.com/notice-regarding-spam-orders/