Following exception happened when fontconfig.properties (fontconfig.bfc) file was used and font setting was NOT configured via fontconfig:
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Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: class sun.font.CompositeFont cannot be cast to class sun.font.PhysicalFont (sun.font.CompositeFont and sun.font.PhysicalFont are in module java.desktop of loader 'bootstrap')
        at java.desktop/sun.font.SunFontManager.getDefaultPhysicalFont(SunFontManager.java:1081)
        at java.desktop/sun.font.SunFontManager.initialiseDeferredFont(SunFontManager.java:960)
        at java.desktop/sun.font.SunFontManager.findOtherDeferredFont(SunFontManager.java:898)
        at java.desktop/sun.font.SunFontManager.findDeferredFont(SunFontManager.java:914)
        at java.desktop/sun.font.SunFontManager.findFont2D(SunFontManager.java:2105)
        at java.desktop/java.awt.Font.getFont2D(Font.java:500)
        at java.desktop/java.awt.Font$FontAccessImpl.getFont2D(Font.java:237)
        at java.desktop/sun.font.FontUtilities.getFont2D(FontUtilities.java:139)
        at java.desktop/sun.font.FontUtilities.fontSupportsDefaultEncoding(FontUtilities.java:316)
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I could recreate by attached testcase on Linux platform.
In case of AIX platform without fontconfig, defaultFontName was defined as "Dialog".

$ $HOME/jdk-14+16/bin/java --add-opens java.desktop/sun.font=ALL-UNNAMED DefaultFontTest
Exception in thread "main" java.lang.reflect.InvocationTargetException
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:564)
        at DefaultFontTest.main(DefaultFontTest.java:47)
Caused by: java.lang.ClassCastException: class sun.font.CompositeFont cannot be cast to class sun.font.PhysicalFont (sun.font.CompositeFont and sun.font.PhysicalFont are in module java.desktop of loader 'bootstrap')
        at java.desktop/sun.font.SunFontManager.getDefaultPhysicalFont(SunFontManager.java:1081)
        ... 5 more

Fixed test result on RHEL7.6 was as follows:
$ java -Dsun.awt.fontconfig=$HOME/fontconfig.properties --add-opens java.desktop/sun.font=ALL-UNNAMED DefaultFontTest
** Type1 Font: Family=Courier 10 Pitch Name=Courier 10 Pitch style=0 fileName=/usr/share/X11/fonts/Type1/c0419bt_.pfb

If Dialog font's slot 0 was missing
$ java -Dsun.awt.fontconfig=$HOME/fontconfig.properties.bad --add-opens java.desktop/sun.font=ALL-UNNAMED DefaultFontTest
** Type1 Font: Family=Bitstream Charter Name=Bitstream Charter style=0 fileName=/usr/share/X11/fonts/Type1/c0648bt_.pfb

Note:
Please change font file name if you are using non RHEL7.6.

JTreg test results for webrev.05 were as follows:
Without FIX
$ jtreg -v DefaultFontTest.java
runner starting test: java/awt/font/DefaultFontTest/DefaultFontTest.java
runner finished test: java/awt/font/DefaultFontTest/DefaultFontTest.java
Failed. Execution failed: `main' threw exception: java.lang.ClassCastException: class sun.font.CompositeFont cannot be cast to class sun.font.PhysicalFont (sun.font.CompositeFont and sun.font.PhysicalFont are in module java.desktop of loader 'bootstrap')
Test results: failed: 1

With FIX
$ jtreg -v DefaultFontTest.java
runner starting test: java/awt/font/DefaultFontTest/DefaultFontTest.java
runner finished test: java/awt/font/DefaultFontTest/DefaultFontTest.java
Passed. Execution successful
Test results: passed: 1

The fix was posted by:
https://mail.openjdk.java.net/pipermail/2d-dev/2019-October/010324.html