Entities not created on Polling import

I am developing a Powershell connector. It uses exactly the same script for Full and Polling imports, just with a "-RunType Delta" switch for polling. The difference is all about how it detects how many users to look at - once it gets to creating the IdB entities the script is identical.

I have four new objects in the external system. They are correctly identified by the script on a Polling import, however the new entities are not created in IdB. When I run a Full import the entities are created. (I have done searches to confirm this)

The following log excerpt shows the four entities that should be created, but the changes reported are 0:

20171211,04:47:22,UNIFY Identity Broker,Logging,Information,Exchange Protected Import: Creating 4 entities,Normal
20171211,04:47:23,UNIFY Identity Broker,Logging,Information,"
Key   : DN
Value : CN=Supressed1\, Changed1,OU=ACT,OU=Users,OU=Accounts,OU=DEV,DC=domain
Key   : PersonNumber
Value : 20523
Key   : TerminationAutoReply
Value : False
Key   : Status
Value : ACTIVE
Key   : HiddenFromGAL
Value : False
",Normal
20171211,04:47:23,UNIFY Identity Broker,Logging,Information,"
Key   : DN
Value : CN=Supressed2\, Changed2,OU=ACT,OU=Users,OU=Accounts,OU=DEV,DC=domain
Key   : PersonNumber
Value : 11831
Key   : TerminationAutoReply
Value : False
Key   : Status
Value : ACTIVE
Key   : HiddenFromGAL
Value : False
",Normal
20171211,04:47:23,UNIFY Identity Broker,Logging,Information,"
Key   : DN
Value : CN=Supressed3\, Changed3,OU=ACT,OU=Users,OU=Accounts,OU=DEV,DC=domain
Key   : PersonNumber
Value : 73564915
Key   : TerminationAutoReply
Value : False
Key   : Status
Value : ACTIVE
Key   : HiddenFromGAL
Value : False
",Normal
20171211,04:47:23,UNIFY Identity Broker,Logging,Information,"
Key   : DN
Value : CN=Supressed4\, Changed4,OU=ACT,OU=Users,OU=Accounts,OU=DEV,DC=domain
Key   : PersonNumber
Value : 18582
Key   : TerminationAutoReply
Value : False
Key   : Status
Value : ACTIVE
Key   : HiddenFromGAL
Value : False
",Normal
20171211,04:47:23,UNIFY Identity Broker,Connector,Information,"Request to import changes from connector.
Request to import changes from connector PowerShell Exchange PROTECTED.",Normal
20171211,04:47:23,UNIFY Identity Broker,Connector,Information,"Import changes from connector completed.
Import changes from connector PowerShell Exchange PROTECTED reported 0 changes. Duration: 00:00:00",Normal
20171211,04:47:23,UNIFY Identity Broker,Change detection engine,Information,"Change detection engine import changes completed.
Change detection engine import changes for connector PowerShell Exchange PROTECTED returned 0 possible changes. Duration: 00:00:04.8439784",Normal

The part of the script the creates the entities is as follows. When I generated that log above I had the two lines uncommented that log the full $entity:

###
### Create/update entities
###
if ($ManagedUsers.count -gt 0) 
{
    $logger.LogInformation("$LogPrefix Creating {0} entities" -f $ManagedUsers.count.ToString())
    foreach ($user in $ManagedUsers)
    {
        $entity = $entities.Create()
        $entity["PersonNumber"] = $user.employeeNumber
        $entity["DN"] = $user.DistinguishedName
        $entity["Status"] = $user.extensionAttribute13
        if ($Mailboxes.ContainsKey($user.DistinguishedName))
        {
            $mb = $Mailboxes.($user.DistinguishedName)
            $entity["DirectPermissions"] = $mb.DirectPermissions
            $entity["HiddenFromGAL"] = $mb.HiddenFromGAL
            $entity["MailboxType"] = $mb.MailboxType
            $entity["PrimaryEmailAddress"] = $user.mail
            $entity["ProxyEmailAddresses"] = $mb.EmailAddresses
            $entity["TerminationAutoReply"] = $mb.TerminationAutoReply
            $entity["TerminationMailCount"] = $mb.TerminationMailCount
        }
        else
        {
            $entity["DirectPermissions"] = $null
            $entity["HiddenFromGAL"] = $false
            $entity["MailboxType"] = $null
            $entity["PrimaryEmailAddress"] = $null
            $entity["ProxyEmailAddresses"] = $null
            $entity["TerminationAutoReply"] = $false
            $entity["TerminationMailCount"] = $null
        }
        #[string]$str = $entity | fl | out-string
        #$logger.LogInformation($str)
        $entity.Commit()
    }
}

I'm on v5.1.0 Revision #1. I should be able to upgrade but it's a bit of a process to get software into the environment, so is there anything else I should be looking at?