Deploying updated content that has been previously translated on the target site drops the translation link.

I have new content on my original site. I have deployed that content to a second site.

On the second site I have translated that content. So far so good. Everything works as expected and the tnid set set correctly on both the original language node and the translated node.

Back on the original site I update the node and re-deploy it. When it arrives at the second site the second site does not recognise that there is a translation for the node in question. It therefore updates the original language node but sets the tnid to 0. It should not be touching this tnid.

As a consequence in node edit mode of the second site it states there is no translation but when I then attempt to add a translation it barfs when I save with the message 'There is already a translation in this language'. Which technically is true as there is an orphan node left behind by the first deploy.