public class CustomConfigSource implements ConfigSource {

    private static final int ORDINAL = 999;
    private static final String PROPERTIES_FILE = "/configsource.properties";

    // ...
}

from.custom.source.person.name=From Config Source
from.custom.source.person.age=33

@Path("/config-mapping")
public class ConfigMappingResource {

    @Inject
    @ConfigMapping(prefix = "from.custom.source.person")
    PersonInterface personFromCustomSourceInterface;

[app] Caused by: io.smallrye.config.ConfigValidationException: Configuration validation failed:
08:56:23.182 INFO  [app]        java.util.NoSuchElementException: SRCFG00014: The config property from.custom.source.person.name is required but it could not be found in any config source
08:56:23.183 INFO  [app]        java.util.NoSuchElementException: SRCFG00014: The config property from.custom.source.person.age is required but it could not be found in any config source
08:56:23.184 INFO  [app]        at io.smallrye.config.ConfigMappingProvider.mapConfiguration(ConfigMappingProvider.java:840)
08:56:23.185 INFO  [app]        at io.smallrye.config.ConfigMappingProvider.mapConfiguration(ConfigMappingProvider.java:796)
08:56:23.186 INFO  [app]        at io.smallrye.config.SmallRyeConfigBuilder.build(SmallRyeConfigBuilder.java:403)
08:56:23.186 INFO  [app]        ... 17 more
The root cause of this issue is this commit fd4b302. Note that this is a regression test that was working fine in 2.1.2.Final.