Using this appender, the files are not deleted. It works without the aux date, but in my case the aux date is very important.

<appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
    <rollingPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedRollingPolicy">
        <fileNamePattern>c:/temp/logs/spring-FILE-boot.%d{yyyy-MM-dd} - %-3i - %d{HH.mm.ss, aux}.log</fileNamePattern>
        <maxFileSize>500KB</maxFileSize>    
        <maxHistory>5</maxHistory>
        <totalSizeCap>1MB</totalSizeCap>
        <cleanHistoryOnStart>true</cleanHistoryOnStart>
    </rollingPolicy>
    <encoder>
        <pattern>%-20(%d{dd.MM.yyyy HH:mm:ss.SSS} [%-20thread]) %-5level %msg [%logger:%line]%n</pattern>
    </encoder>
</appender>