You need to set AsyncAppender's includeCallerData property to true. Here is the modified config file:
<configuration>
<appender name="FILE" class="ch.qos.logback.core.FileAppender">
<file>myapp.log</file>
<encoder><pattern>%logger{35} - [%F:%L] - %msg%n</pattern></encoder>
</appender>
<appender name="ASYNC" class="ch.qos.logback.classic.AsyncAppender">
<appender-ref ref="FILE" />
<!-- add the following line -->
<includeCallerData>true</includeCallerData>
</appender>
<root level="DEBUG"><appender-ref ref="ASYNC" /></root>
</configuration>