This problem, as frostfreeedet said, has been around for a long time. The programming seems to look at the father's death date more accurately than it does the mother's. Usually, if I go to the actual mother's profile to add a child, I can add the child without any warning. Adding a child from a father only works if the father only has one wife. If you don't have any dates of death entered for the parental profiles involved, the warning won't display for the most part.
This issue is a little bit of a pain point in the online tree program here, but nothing I can't work around. Others may have different opinions or experiences but the above has been mine.