Using Deploy module, translated nodes lose their translation links (tnid)

When used with the Deploy module, content that has been previously translated loses its tnid reference and is treated as new content.

For example let's say after the initial node is deployed you get this on the target site:
node 1234 has nid 1234 tnid 0

When the node is subsequently translated on the target site you get this:
node 1234 has nid 1234 tnid 1234
node 1235 has nid 1235 tnid 1234

So far, so good but...

Later, when the content on the original site is updated and deployed:
node 1234 has nid 1234 tnid 0 <--see how it has lost its translation link?
node 1235 has nid 1235 tnid 1234 <-- this remains unchanged as expected

This has two negative effects:
1. The translation 'link' is lost.
2. You cannot then translate it as you get an error saying the content has already been translated.