Hi, I think that you have found the problem 
If I put "due:19700101" in my regular search bar, tasks without due dates appear.
If I put "due:19700201" tasks without due dates don't appear.
I thought that it was just a typo, and that they both worked the same, but I just found this:
"In Joplin, a task without an assigned date has one corresponding to 1970-01-01"