Some key tasks that appear on the Task List do not appear on Gantt chart however you can see they are hidden as you can see on the Gantt schedule a message: 'Hidden/Off Screen Predecessors: (Task Name)'.
Key tasks were not getting displayed in the Gantt and this turned out to be caused by the 'Key Task' attribute in the Gantt filter section
which has display type 'Check Box'.
A checkbox has two states: checked or unchecked. In the case of a filter when a boolean attribute is defined as a checkbox this means that you are asking for either values that are true or false.
The behavior of Show All on a filter is supposed to clear all the values from a filter and then issue the filter action. When a boolean defined as a checkbox is on the
filter, the Show All will clear the checkbox (uncheck it) and issue the filter. What this means in the filter is that any boolean attribute displayed as a checkbox will
be filtered for the unchecked value (false) during a Show All. This is the way it works regardless of the screen: Gantt, Project List, Task List, etc.
When a boolean attribute is defined as a pull-down in the filter, the attribute has three states: undefined, true, false. The undefined value is mapped to "All" in the pull-down.
This undefined value will cause instances where the attribute value is either true or false to be returned.
It's the fundamental difference between the pull-down display type and the checkbox display type for a boolean attribute.