You're right - I typed wrong info about isnum/istext values.
Attached is sample csv.
As you can see in my case the AST is written as "". (two quotation marks, nothing between them).
Finally, I found the reason of this issue.
From Qlik Help:
"A string with length zero is not considered as a NULL and will cause IsNull to return False."
Nevertheless, it's ridiculous that isnull() doesn't work in that case.
There is no water in the glass, but the glass is not empty. - good one Qlik!
In this case:
In case 1 isnull(value) returns True
In case 2 - False!!!
In case 3 - False
Qlik team has a lot to work on....
test_data2.csv 1.2 K