Looks like Prosper found the payment...
Dear XXX,
We only get the file from the collection agency once a month, you will see your share of this payment show up in your account within the next week.
For future reference, your Prosper support case number is XXXXXXX.
Sincerely,
Prosper Customer Support
This still doesn't explain why if the payment was done on February 28th, why it takes until April 2nd to have the payment start processing. What happened to March?

According to this the "Account effective date" is March 29, but I posted here and emailed Prosper on March 30th that nothing had been processed.
Still waiting to actually get the money, but at least Prosper is acknowledging that there is money that's due.