Some clients don't provide an option to specify the attribute to be read but uses a default selection of attributes that are required for the purpose of the client.
If the client is only used/designed to collect data from an OPC UA server, this may include information about the value (like ValueRank, ArrayDimensions, ...) but knowledge about other attributes (for example WriteMask) is not necessary or even valuable.

So the test case should be updated in a way that the client can limit the supported attributes:
"If the client allows to specify the attribute, then ..."

Or the expected result should be updated:
"Verify the client does read all specified/supported attributes ..."