If an account has too many open orders with a gross value of less than 0.01XBT, the account will be labelled as a spam account and all proceeding orders that are smaller than 0.01XBT in size will automatically become hidden orders.


Spam accounts are re-evaluated every 24 hours and may return to normal provided that trading behaviour has changed:

https://blog.bitmex.com/api_announcement/change-in-spam-order-size-threshold/