Inconsistent tag behavior in smart mailbox

I added a tag "WorkingPapers" to five emails. I then added another tag "wp" to three of those five emails. Let's call the two emails with only "WorkingPapers" type A, and call the emails with "WorkingPapers" and "wp" type B. For these five emails, there are no other user-defined tags (I checked with "Raw Flags").

In the "Tagged" smart mailbox,

• In the "WorkingPapers" submailbox, there are two type-A emails and two type-B emails. The mailbox count is four.
• In the "wp" submailbox, there are two type-B emails. Both also appear in the "WorkingPapers" submailbox. The mailbox count is two.
• There is an "Other" submailbox containing one type-B email, which does not appear in the other two submailboxes. The mailbox count is one.

This behavior seems odd and inconsistent, and I assume it is a bug.

What fixes the problem for me:

• toggling the "Edit Mailbox" (e.g., by switching from "tags" to "tags/keywords" and back) does not fix the problem.
• quitting mailmate and restarting fixes the problem. That is, after restarting, there are: --- two type-A emails and three type-B emails in the "WorkingPapers" submailbox. --- three type-B emails in the "wp" sub mailbox.

Some further observations:

• All emails seem tagged correctly, based on the "Raw Flags" column.
• The email count for the "Tagged" folder is correct, i.e., it accounts for five emails.