I have a tool tip plugin that shows the overset / overmatter of a text frame. The plugin works most of the time except under these conditions.
Launch InDesign, open a document that has a table with a cell that has gone completely into overset, no text showing. hover the mouse over the cell, nothing shows.
If I launch InDesign and open the same document and click anywhere on the page (not in any text frame or cell) and then hover the mouse over the cell, text is displayed. It is as if by clicking into the page some thing is being initialised.
If I remember correctly, table cell overset is not stored in the same way as "normal" overset. You have to iterate the table model to find it.
My point is that InDesign may not actuallty calculate overset on cells it before something triggers it to do this - like clicking on the page. Have you tries to iterate the table right after the document is opened ?