Say we have this schema snippet:

<xs:element name="SimpleDataFormat">
    <xs:complexType>
        <xs:sequence>
            <xs:element name="NumStudents" type="xs:nonNegativeInteger" 
                dfdl:textNumberCheckPolicy="strict"
                dfdl:textNumberPattern="#,###"
                dfdl:textStandardGroupingSeparator=","
                dfdl:textStandardDecimalSeparator="."
            />
        </xs:sequence>
    </xs:complexType>
</xs:element>

This successfully parses the data

1234

Even though textNumberCheckPolicy="strict" and the pattern contains a grouping separator, it still allows data that does not contain grouping separators.

That said, we have generally tried to make DFDL's spec match the behavior of the ICU library for parsing numbers based on the textNumberPattern. This library has this to say about strict parsing of numbers:

The following conditions cause a parse failure relative to [lax] mode
(examples use the pattern "#,##0.#"):

• The presence and position of special symbols, including currency,
  must match the pattern.

'+123' fails (there is no plus sign in the pattern)

• Leading or doubled grouping separators

',123' and '1,,234" fail

• Groups of incorrect length when grouping is used

'1,23' and '1234,567' fail, but '1234' passes

• Grouping separators used in numbers followed by exponents

'1,234E5' fails, but '1234E5' and '1,234E' pass ('E' is not an
exponent when not followed by a number)

So based on ICU's description of strict, this is the expected behavior. It doesn't say anything about missing grouping separators causing an error. Only that if they do exist then they must be in the right spot.

The only thing the DFDL specification mentions regarding strict numbers is this:

If 'strict' and dfdl:textNumberRep is 'standard' then the data must
follow the pattern with the exceptions that digits 0-9, decimal
separator and exponent separator are always recognised and parsed

To me, that reads like the decimal separator should always be required in strict mode, so this feels like the ICU behavior and the behavior described in the DFDL specification do not match. And I believe the DFDL behavior was intended to match ICU behavior, so it's possible the DFDL specification needs to be updated.