It appears that Xinerama and Xrender support has never ever been functional on AIX.

The fixed version can be tested as follows:

Simulate a multi-screen environment on my Linux box with Xephyr:

$ Xephyr +xinerama -screen 1024x768 -screen 1024x768+1024+0 -ac -listen tcp :1

Started a Swing application from AIX where you redirect the DISPLAY to myhost:1:

$ DISPLAY=myhost:1 ./images/jdk/bin/java -showversion -Dsun.awt.nativedebug=true -Dawtdebug.trace=true -Dawtdebug.on=true -Dawtdebug.ctrace=true -cp ~/Java HelloSwing

The various debug properties will lead to the following output:

openjdk version "13-internal" 2019-09-17
OpenJDK Runtime Environment (slowdebug build 13-internal+0-adhoc.xxx.jdk-jdk)
OpenJDK 64-Bit Server VM (slowdebug build 13-internal+0-adhoc.xxx.jdk-jdk, mixed mode)
Xinerama extension is available
calling XineramaQueryScreens func
Enabling Xinerama support
 num screens = 2
allocating 2 screens

The Swing frame will appear on one of the Xinerama screens and can be move with the mouse between the two available screens.