This issue from https://bugzilla.redhat.com/show_bug.cgi?id=1300439.
It was fixed for JBoss EAP 6.4 but the fix needs to be applied to topic 5701.adoc "Example IBM DB2 XA Datasource" for JBoss EAP 7 also.

Describe the issue:
The DB2 driver will attempt to behave as a JDBC Type 2 driver unless <xa-datasource-property name="DriverType">4</xa-datasource-property> is specified. Please see the below example.
Raw

<subsystem xmlns="urn:jboss:domain:datasources:1.2">
<datasources>
...
<xa-datasource jndi-name="java:jboss/xa-datasources/testPool" pool-name="testPool" enabled="true">
<xa-datasource-property name="ServerName">localhost</xa-datasource-property>
<xa-datasource-property name="PortNumber">50000</xa-datasource-property>
<xa-datasource-property name="DatabaseName">SAMPLE</xa-datasource-property>
<xa-datasource-property name="DriverType">4</xa-datasource-property>
<driver>db2jcc4.jar</driver>
<xa-datasource-class>com.ibm.db2.jcc.DB2XADataSource</xa-datasource-class>
<xa-pool>
...
Additional information: https://access.redhat.com/solutions/2132861