I don't know why it is, or why LC thinks it's acceptable, but all extra payments - anyone who pays in full or pays extra on their principal - take a while to get through the system. Maybe these payments have to be processed manually and there is a backlog? I dunno. Also, it is typical to see them credit one "normal" payment out of the larger payment the borrower made, and then the rest of the money shows up later in a separate payment.
I have a note that paid $17 and change with a completion date of 3/15. 3/25 I got $11.02 and a separate payment of $5.89 (I have no idea why they thought that made sense). Another one had a completion date of 3/18 and payment arrived 3/26. So they always have a delay.
That said, a month (or nearly a month) seems too long to me and I would write LC an e-mail and ask about it.