This line here:

https://github.com/qos-ch/slf4j/blob/master/slf4j-jdk-platform-logging/src/main/java/org/slf4j/jdk/platform/logging/SLF4JPlatformLogger.java#L145

message = String.format(message, params);

Should instead be:

message = MessageFormat.format(message, params);

Such that it matches the expected format of System.Logger.log() ... as per https://docs.oracle.com/javase/9/docs/api/java/lang/System.Logger.html#log-java.lang.System.Logger.Level-java.lang.String-java.lang.Object...-