The mogrify tool exported the *.class files but not the *.properties file.
15.0 Identity Suite
This behavior can be reproduced using mogrify-15.0.0-147-fixpack3, which is the latest version at the time this article was written.
The workaround to this problem was:
Manually copy the .properties files skipped by mogrify into the /opt/brcm/iga/conf/idm/classes so that starting IM triggered them to be copied into the /opt/brcm/iga/inst/wildfly-idm/standalone/deployments/iam_im.ear/custom folder as long as those files were owned by the igx user.