Set loses all translations

How to reproduce:
* create a new page node: language: 'German', title: 'German test page', URL path: 'testpage'
* for that newly created node, create a translation: language: 'English', title: 'English translation for test page', URL path: 'eng/testpage'
* goto the german translation (node created at the very beginning), edit that node, make some arbitrary changes and save that node.
* now click on the translation tab for that node: while I would expect, that the English translation is still present, no translations are present any more. If you created more translations than the English one, all of them are removed from the set.