After upgrading from eZ Publish 4.5 to 4.6, the content of a previously created block-style custom tag such as 'factbox' disappears and is replaced by the name of the custom tag in question.
1. Create a clean eZ Publish 4.5 installation;
2. Create an object of type 'article', and on an XML block field ('body' will do) add a block custom tag and place some content inside. In my case, I used a factbox and placed a few PDF links inside. Here is the resulting code, obtained after disabling the editor:
3. Upgrade to eZ Publish 4.6 (standard upgrade, described in documentation - http://doc.ez.no/eZ-Publish/Upgrading/Upgrading-to-4.6/Upgrading-from-4.5-to-4.6);
4. Go to the article and check the previously created factbox. Its original content is gone, and it now contains its own name ('factbox') instead. Here is the underlying code: