log4net traceappender only logs messages with level 'verbose' when using Windows Azure DiagnosticsMonitor

I have an azure worker role which I have configured to use a log4net Trace Appender which writes to WindowsAzure.Diagnostics. This is done by making the following calls in the RoleEntryPoint of the worker role.

using System;
using Microsoft.WindowsAzure.Diagnostics;
using log4net.Config;

namespace XXX
{
    public class WorkerRole : RoleEntryPoint
    {
        public override bool OnStart()
        {
            var config = DiagnosticMonitor.GetDefaultInitialConfiguration();

            config.Logs.ScheduledTransferLogLevelFilter = LogLevel.Warning;
            config.Logs.ScheduledTransferPeriod = TimeSpan.FromMinutes(5);

            config.WindowsEventLog.DataSources.Add("System!*");
            config.WindowsEventLog.DataSources.Add("Application!*");
            config.WindowsEventLog.ScheduledTransferLogLevelFilter = LogLevel.Error;
            config.WindowsEventLog.ScheduledTransferPeriod = TimeSpan.FromMinutes(5);

            DiagnosticMonitor.Start("Microsoft.WindowsAzure.Plugins.Diagnostics.ConnectionString", config);

        XmlConfigurator.Configure();
        }
    }
}

The App.config file is configured in the following manner:

<log4net>
    <appender name="TraceAppender" type="log4net.Appender.TraceAppender">
        <layout type="log4net.Layout.PatternLayout">
            <conversionPattern value="%logger - %message" />
        </layout>
    </appender>
    <root>
        <level value="ALL" />
        <appender-ref ref="TraceAppender" />
    </root>
</log4net>

<system.diagnostics>
    <trace>
        <listeners>
            <add type="Microsoft.WindowsAzure.Diagnostics.DiagnosticMonitorTraceListener, Microsoft.WindowsAzure.Diagnostics, Version=1.7.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" name="AzureDiagnostics">
                <filter type="" />
            </add>
        </listeners>
    </trace>
</system.diagnostics>

The result of this is that all messages (even errors) are logged in table storage as "verbose" level.

How to fix this?