What has project.build.sourceEncoding got to do with resources?

Maven Resources Plugin has, up until version 3.2.0, defaulted to use project.build.sourceEncoding as the encoding when filtering resources, unless you configure the encoding parameter of the plugin explicitly. So unless you have configured the encoding parameter in Maven Resources Plugin explicitly this is what you get.

Properties files handled by the Properties class

When the Properties class is used to read and write properties files they require that the properties files use ISO-8859-1 encoding. This is still the case for Java 11, as can be seen in the API documentation for the Properties class. So, properties files that are used in this way needs to use ISO-8859-1 encoding.

Properties files used as ResourceBundle

When properties files are used as ResourceBundles the encoding required differs between versions of Java. Up to and including Java 8 these files are required to use ISO-8859-1 encoding.

Starting with Java 9 the preferred encoding is UTF-8 for property resource bundles. It might work with ISO-8859-1, but as you can see in the Internationalization Enhancements in JDK 9 documentation you should consider converting your property resource bundles into UTF-8 encoding.

What do I need to do?

You need to do 2 things:

1. Decide which encoding to use for properties files, based on how you use them in your project.
2. Explicitly configure Maven Resource Plugin accordingly using the propertiesEncoding configuration parameter, that was introduced in version 3.2.0. In most cases it would look like this:

   <project>
     ...
     <build>
       <plugins>
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-resources-plugin</artifactId>
           <version>4.0.0-beta-1</version>
           <configuration>
             ...
             <propertiesEncoding>ISO-8859-1</propertiesEncoding>
             ...
           </configuration>
         </plugin>
       </plugins>
       ...
     </build>
     ...
   </project>