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Row ID must be monotonically increasing for the new rows. Tableau will only incrementally insert any new rows whose Row ID is greater than all previously inserted rows, not just different from all previously inserted rows. If the first 39 rows you grabbed were random, then their Row IDs were too, which explains why you got some but not all new rows the next time, and nothing after that.
Ahhh, that was it.
Sorted by RowID in Excel and re-did the experiment and it now works.
Tableau only picking up new records if the column identifier is increasing is not in the extract documentation at all. Yikes.
Thanks for the help!
Excellent. Point 3 in this help article does imply the row IDs must be increasing, but it's easy to miss.