The short answer is that is is working as designed.
The SUBSELECT functionality was developed by Qlik as an addition to the standard SAP OPEN SQL. It makes it possible to do sub selects (like JOINS) of cluster tables, which is not possible in the standard SAP OPEN SQL. It does not support everything that is available in the SAP OPEN SQL. So for the moment it is not possible to use 'IN' conditions is the SELECT of the sub table. Neither it is possible to use AND in the the SELECT of the main table.
Here is a link to the online help where the SUBSELECT is described: