I am, like David Abernathy, unable to duplicate your problem in FTM2014. I find that in an order from oldest to most recent, facts with blank dates sort beneath facts with dates - so burial is always sorting below death fact with a date - even if the burial date is blank - EXCEPT, if I choose to sort alphabetical by Fact Name. (That is accomplished by clicking on the header of the Fact Name column) - where burial sorts before death - or by sorting by date most recent on top to oldest on bottom - then blank dates sort to the top.
This is true whether in the Facts Mode or the Timeline mode.
Are you using FTM2014 or a pervious version?
Where is it that you are seeing something different?
1) Timeline or Facts view?
2) Which column have you selected as primary and secondary sorts (by clicking on headers)?