There is no 'rain rate' parameter defined in the WU protocol. The protocol requires the total rain so far today, and the rain in the last hour. The column that WU call "rain rate" is actually the rain in the last hour.
I don't know why WU isn't showing anything for 'rain last hour'. The code in Cumulus correctly sends the 'rain last hour' that it calculates, so either WU isn't showing it, or for some reason the rain last hour calculation isn't working on your system. Here's part of my WU from earlier today when I last had some rain:
2013-03-29 18_03_19-Weather Station History _ Weather Underground.png
You would normally expect to see something in what WU calls "rain rate" if you are seeing something for "rain total", as you are in the examples you quote, I agree.
I would need to see the debug log at the time when it was raining so that I could see what Cumulus was sending to WU, and you could see what Cumulus was showing for 'last hour' on the rain section on the main screen. You're using 'rapid fire', so there will be a lot of entries in the debug log very quickly, but you only need to leave it on for a couple of minutes.
Was that data sent while Cumulus was running 'live', or had you just started Cumulus up and it was sent as 'catch up'? Perhaps there's a bug in the 'rain in last hour' calculation when it's using logger data.
Edit: As it happens, that data of mine this morning was sent using logger data, so I can confirm that it does work (usually):
weatherstation.wunderground.com/weatherstation/updateweatherstation.php?ID=IORKNEYS2&PASSWORD=*********&dateutc=2013-03-29+07%3A10%3A00&winddir=67&windspeedmph=18&windgustmph=18&windspdmph_avg2m=18&winddir_avg2m=68&humidity=72
&tempf=37.7
&rainin=0.01&dailyrainin=0.01&baromin=30.070&dewptf=29.5&solarradiation=109
&softwaretype=Cumulus%20v1.9.4&action=updateraw