Hi, Cameron!
generally speaking, it is up to application developer's discretion how to deal with orders marked as "Entered in Error". I would expect that for majority of use cases an application would filter those out. However, there may be scenarios where showing "Entered in Error" might make sense within the application workflow. The API provides an option to filter data by status on the query [1] and of course a developer may choose to post filter this data.
To answer your question about the meaning of this status, we have to create concept mappings for the 10+ status code values that are routinely used by Millennium medication orders to match up to the 4 status codes offered by the HL7 spec [2]. In this case for an outbound query, we map Millennium code values of "Deleted" & "Voided with Results" to a single FHIR status code of "Entered in Error".
In particular example you are bringing up of Apple Health, I will reach out to them to see if they can clarify.
Thanks,
Yegor (Cerner)