Non-visible dates and date:overdue showing items with no apparent date

In a document with this structure, I am seeing some strange filtering behavior with date:overdue

It gives me this:

There are no dates at all in the document. If I add an explicit overdue, then it also shows up.

If I add today's date to Heading 1, now the filter excludes them all. It's as if an invisible overdue date is on Heading 1. (note that although no date is there now, there was one there in the past)

After that, removing the date on Heading 1, overdue correctly does not show anything. So the action of adding a date and removing it "fixed" the item.

Looking at the json for Heading 1 (via export before I started messing with it)

    {
      "id": "-O_PiSmaTG51zK8ULi5d",
      "i": "0",
      "t": "# Heading 1 ",
      "b": "c:km1ge4s1usfbb56uj187k1lu18;b:861fa41a81019a05e92089aa45a6d64ff76d24b11ca6cd4fed7891ebeba5775e@group.calendar.google.com\\1758250800000",
      "c": 1758163754318,
      "q": false,
      "p": "-O_PiSm_u-PhHz86Q1C-"
    },

I strongly suspect our culprit is data value "b" which seems to encode a date and/or calendar reference (props for obscure naming of values, by the way). When I added a date to another item I saw "b" being populated. However it's very unclear why nothing is showing up.

So it seems to be that there is a non-visible date associated with the item.