This problem has now been fixed ! (with very impressive speed)
As of Sneaky Peek build revision: 135625, the following now works well:
Code:
tell application "OmniFocus"
tell front document
set refTasksNoStart to a reference to (flattened tasks where start date is missing value)
set refTasksNoDue to a reference to (flattened tasks where due date is missing value)
set refProjectsNoStart to a reference to (flattened projects where start date is missing value)
set refProjectsNoDue to a reference to (flattened projects where due date is missing value)
end tell
end tell