This is not really an incorrect warning message.
1. Machines are only as smart as the data they have access to. In this case the data provided is simply '1869 < 1871 = show message'. "After" doesn't mean a whole lot to me, much less a machine/code. Does After 1869 mean 1870? 1871? 1899? 1950? All of the above? What are the bounds of that assumption?
2. This warning is more of a "Check your data" type of message. Something doesn't look quite right to the system so it is asking you for help and clarification - did you perhaps fat finger a date? Is this the correct person you are attaching this data to?
If you have found a source that proves the relationship, then you have also found a source that proves an update to the mother's date of death! While the exact date is still not known, you can modify it to be "After 1871" citing the child as the source.