Exchange 2013 CU7\8 Activesync ignores retention setting and soft deletes from device around midnight

We're running an Exchange 2013 CU7 (upgrading to CU8) and we have some users who have emails on their ios device set to retain emails for 1 month. Despite this, around midnight all but the previous weeks email are soft deleted from the their device. We tried with different devices using different activesync gateways (MobileIron and Blackberry) and experienced the same thing each time. Microsoft support have been less than helpful, has anyone else seen this?

<Sync xmlns="AirSync:">
<Collections>
<Collection>
<SyncKey>2111111110</SyncKey>
<CollectionId>1</CollectionId>
<Status>1</Status>
<Commands>
<SoftDelete>
<ServerId>1:69</ServerId>
</SoftDelete>
<SoftDelete>
<ServerId>1:80</ServerId>
</SoftDelete>
<SoftDelete>
<ServerId>1:89</ServerId>
</SoftDelete>
<SoftDelete>
<ServerId>1:101</ServerId>
</SoftDelete>
</Commands>
</Collection>
<Collection>
<SyncKey>11111111117</SyncKey>
<CollectionId>25</CollectionId>
<Status>1</Status>
<MoreAvailable/>
<Commands>
<SoftDelete>
<ServerId>25:726</ServerId>
</SoftDelete>
<SoftDelete>
<ServerId>25:727</ServerId>
</SoftDelete>
<SoftDelete>
<ServerId>25:728</ServerId>
</SoftDelete>