I am now using the Windows app with Due Today sync, but most of my tasks were acquired from a Toodledo sync. When I mark one of the repeating tasks acquired from the Toodledo sync as completed by ticking it in the list of tasks it gets permanently deleted and I have to go to the completed tasks list to bring it back to life. However if I delete it by first double-clicking on the task it seems to behave properly.
It may be relevant that I have two tasks of the same name, but in different projects. This came about because at some point I marked the original task as completed in the Android client and it got duplicated on the Toodledo server; I then edited one version of the task.
When I create a new repeating task directly in the Windows app and I tick it as completed in the list of tasks, it comes back as it is supposed to.