Queries :: Query With Instr Returns Unexpected Results

I have Access 2010. If I run this query

Code:
SELECT Reference,
InStr(1,FunctionThatReturnsLongString(), ResponsiblePerson) As MyField
FROM MyTable
WHERE
(Reference ='ShouldBeOut'
OR
Reference = 'ShouldBeIn1'
OR
Reference = 'ShouldBeIn2')

It returns 3 rows, with values in MyField of 0, 23 and 355.

Now I add 1 more where clause to filter on MyField and the query looks like this:

Code:
SELECT Reference,
InStr(1,FunctionThatReturnsLongString(), ResponsiblePerson) As MyField
FROM MyTable
WHERE
(Reference ='ShouldBeOut'
OR
Reference = 'ShouldBeIn1'
OR
Reference = 'ShouldBeIn2')
AND
(
InStr(1,FunctionThatReturnsLongString(), ResponsiblePerson) > 0
)

So, you would think that it should return 2 values (ShouldBeIn1 and 2), but it doesn't. It only return the value where the Instr returns value of 23. The one with the value of 355 also disappears. Why would that be? Surely Instr does not return a byte?