Ok everyone. I figured out the solution. Thanks much for all the great suggestions. Most of them I had already tried before I decided to post here. So the sanity check was of great value!
For whatever illogical reason, what fixed this was wrapping the field in CInt() even after the field was confirmed already to have been numeric. It would not allow a Sum() to run against the field without it. Here's the code that finally took:
=Sum(
IIF(
IsNumeric(Fields!ADataField.Value) And
DatePart("q", Fields!createdon.Value) = "1",
CInt(Fields!ADataField.Value),
0
)
)
Any and all other permutations that we've all tried here, including those with RunningValue() all work after having wrapped the "true" portion of IIF() with CInt().
Just please don't axe me why this works and yet it doesn't without it. Anything but the most elementary expressions always seem to be extremely fickle.
Thanks again everyone. Answered question goes to GShenanigan for going the extra mile during chat. Wish I could give it to everyone though. You guys are awesome.