Log4j RollingFileAppender has odd behavior when using the Log4j 1.x bridge

This is a bug and you should report it. The RollingFileAppenderBuilder (cf. source code) has a filePattern containing a date instead of a number:

        String filePattern = fileName +"%d{yyy-MM-dd}";
        TriggeringPolicy timePolicy = TimeBasedTriggeringPolicy.newBuilder().setModulate(true).build();
        SizeBasedTriggeringPolicy sizePolicy = SizeBasedTriggeringPolicy.createPolicy(maxSize);
        CompositeTriggeringPolicy policy = CompositeTriggeringPolicy.createPolicy(sizePolicy, timePolicy);
        RolloverStrategy strategy = DefaultRolloverStrategy.newBuilder()
                .setConfig(config)
                .setMax(maxBackups)
                .build();
        return new AppenderWrapper(RollingFileAppender.newBuilder()
                .setName(name)
                .setConfiguration(config)
                .setLayout(fileLayout)
                .setFilter(fileFilter)
                .setBufferedIo(bufferedIo)
                .setImmediateFlush(immediateFlush)
                .setFileName(fileName)
                .setFilePattern(filePattern)
                .setPolicy(policy)
                .setStrategy(strategy)
                .build());

and hence it behaves more like the DailyRollingFileAppender, then a Log4j 1.x RollingFileAppender.

As a workaround you can use the older Log4j1ConfigurationFactory by setting these system properties:

log4j2.configurationFactory=org.apache.log4j.config.Log4j1ConfigurationFactory
log4j2.configurationFile=classpath:log4j.properties