System Property Substution does not work for max-pool-size in datasource xml

I have changed the esb data sources to use system properties.
Properties are loaded from a file via jboss-service.xml via
<mbean code="org.jboss.ejb.plugins.cmp.jdbc.metadata.MetaDataLibrary"
name="jboss.jdbc:service=metadata"/>

<mbean code="org.jboss.varia.property.SystemPropertiesService"
name="jboss:type=Service,name=MySystemProperties">

<attribute name="URLList">
machine.properties
</attribute>
</mbean>

All properties work, except ESB_ORACLE_POOL_SIZE.
It doesn't generate an error it simply reverts to minimum pool size value.

<datasources>
<local-tx-datasource>
<jndi-name>JmsDS</jndi-name>
<connection-url>${ESB_ORACLE_URL}</connection-url>
<driver-class>oracle.jdbc.driver.OracleDriver</driver-class>
<user-name>${ESB_ORACLE_USER}</user-name>
<password>${ESB_ORACLE_PWD}</password>
<transaction-isolation>TRANSACTION_READ_COMMITTED</transaction-isolation>
<exception-sorter-class-name>org.jboss.resource.adapter.jdbc.vendor.OracleExceptionSorter</exception-sorter-class-name>
<min-pool-size>15</min-pool-size>
<max-pool-size>${ESB_ORACLE_POOL_SIZE}</max-pool-size>
<prepared-statement-cache-size>50</prepared-statement-cache-size>
<share-prepared-statements>true</share-prepared-statements>
<metadata>
<type-mapping>Oracle9i</type-mapping>
</metadata>
</local-tx-datasource>
</datasources>

#ybsoa
ESB_ORACLE_USER=YBPRDESB
ESB_ORACLE_PWD=.......
ESB_ORACLE_URL=jdbc:oracle:thin:@(DESCRIPTION = (LOAD_BALANCE = yes) (ADDRESS_LIST = (ADDRESS = (PROTOCOL = TCP) (Host = vip-ybkopracdb04.corp.ybusa.net) (Port = 1521))(ADDRESS = (PROTOCOL = TCP)(HOST = vip-ybkopracdb05.corp.ybusa.net)(PORT = 1521)))(CONNECT_DATA=(SERVICE_NAME = ESB)(SERVER = DEDICATED)))
ESB_ORACLE_POOL_SIZE=100