Thanks for the report.
There is indeed an issue with txt export in that case, we are going to fix it.
Explanation of this behavior : when people put a lot of objects without any links, GraphViz uses some horizontal layout behaviour. To force a more compact layout, PlantUML silently adds some "ghost" links between objects. Those links are supposed not to be printed (in PNG for example). As you figure out, those links are wronly displayed with txt export.