I completed some repeating tasks on my iPhone and on the iPhone, the task regenerated correctly. Then did a sync. The tasks have not been marked as "Completed" on Toodledo web, and they therefore did not regenerate with the new due dates.
I repeated the sync on the iPhone again. No change. bug????
I just tried, and I can't reproduce this. I am running a beta of the next version, so it is possible this is something that has been fixed. It still might be a good idea to start a support ticket. If you do start a ticket, you should probably state whether you were using autosync, sync on startup, or manually syncing, and any other relevant details.