The following formula works if it is entered as an array formula, but
it also works if it is entered conventionally:

=SUMPRODUCT(--(U$1:U$382=A390),--(ISBLANK(AL$1:AL$382)))

However, this formula ONLY works if it is entered as an array formula:

=SUM(--ISBLANK(AL2:AL382))

Presumably there is some logic to this distinction.

What am I missing here?