Bug with Private Message Limits?

Is the following problem a known bug? (this is running PunBB 1.2.17):

Let's say there are 3 member levels, each with the following message limits:

Newbie     50
Member  100
BoD       200

If the BoD has a mailbox 51% full (more than 100 messages), then Newbies and Members can't send PMs to the BoD user (they get a "Users inbox is full, you can not send you* message to the user." response).  If the BoD deletes some messages, so that the % drops below 50%, then the Member can send PMs, but the newbie still can't.  And once the BoD's messages creep up past the 50% level, the problem reappears.

So effectively, the system treats the # of messages in the RECEIVER's mailbox and compares it against the number allowed in the SENDER's mailbox.  If the number in the receiver's mailbox is more than the sender is allowed, then no messages can get through.

At least that's is how it seems to be happening for me.  It's a pretty time-consuming process to test out (creating users, creating and deleting messages, etc.), so I'm not 100% certain this is what's going on, but it seems to be playing out this way.  So that's why I'm posting here, to see:

1) If this is a known problem?    and if so
2) Is there a fix?

If my theory is correct, having a high message limit for a certain level isn't helpful if having too many messages in your mailbox (even though your mailbox isn't full) blocks those in lower levels from sending you any messages.

Thanks!

* SIDE NOTE: This "send you message" is a typo I've seen on multiple PunBBs, somebody should fix it to read "send your message".