Sleeper: ChartSheet Generates Bogus External Reference

[MWE]

This is not really an Excel VBA problem (I think), but it is certainly an interesting and quirky problem ...

I have an Excel spreadsheet with several sheets and a few graphs on each sheet. There is also a modest amount of VBA code (~ 200 lines). There are no references to any external workbooks.

If I create a Chartsheet from any of the current data, save the spreadsheet and reopen it, an "Update External References" window appears pointing to a non-existent file. I bypass that, change the location of the chartsheet to any worksheet, save the file, reopen the file and the Update External References window does not appear. If I change the location of the chart back to a separate chartsheet, ..., the bogus window appears.

I created a new workbook and moved each worksheet to the new workbook. If I try to create a chartsheet, same problems

Any idea what might be happening here?

[BlueCactus]

Hmmm. Sounds similar to what happens when you copy a chart and its source data between workbooks without telling the duplicate chart to collect the data from the [destination] book that contains it. (Because Excel keeps the source data as the original book.)

I don't know why it would happen within a single book, though.