Advanced search for unset custom date value

I know there is no option to search for unset/null values, but I have a different problem.

We have custom field for tasks defined as:

 "mireo:due-date": {
    "name": "Due Date",
    "type": "date",
    "required": false,
    "search": true

In the dashboard, we have a saved custom query with Due Date Before: now.

  • If someone sets a Due Date, search works as intended (does show up if in past)
  • If Due Date is not set, search works as intended (does not show up)
  • If sets Due Date, and then unsets it (unticks the checkbox Due Date), system treats it as set to the previous value - we have no way to set it to null/nonvalue. It still shows up in dashboard.

Is there a way around it?