Gantt Chart Bars Drawn Inconsistently

When bringing up a Gantt chart with a record which has two or more date fields sometimes the chart shows a simple milestone, other times it shows a bar, making the assumption (incorrectly depending on circumstances) that one of the dates is a start date and another is an end date.

The inconsistency is that it will draw differently for the same record with no rhyme or reason.  This is difficult to recreate.