XIRR() producing accurate return?

Hello. I have a series of non-periodic cashflows, beginning with an investment (expressed as a negative), followed by returns (dividends, expressed as positives) and further investments, culminating in a final return. The net of the cashflows is positive.

The XIRR() function tells me that ROI is 1245%, and in trying to verify the accuracy of this figure, I played with the calculation range, shortening it to exclude the final large return -- yet Excel continues to produce a 1245% ROI. I further shortened the range, one transaction at a time, yet the ROI doesn't change.

In the attached sample, note the dates and values in columns B and C, and the XIRR() function at the bottom, displaying 1245%. Also notice that I've run the same formula in column D, using an absolute reference for the top row in order to calculate a separate ROI with each added transaction. Also please notice the mostly unchanged ROI's in D.

Finally, I replicated the series in columns G & H, but for the final 12/31/18 return, I have increased the value 1000-fold, from $248K to $248M, with no change to the ROI produced by the XIRR() function.

Am I doing something wrong? Is there a better function to use for such a series of cashflows?

Many thanks for any ideas!
Chris