Good news! This appears to be fixed. This morning I attached PDFs to two transactions in my checking register, and they open in a new browser tab when I click the links.
So I guess my suspicion was correct all along: This was a bug introduced with the November 6 update, and now it has been corrected in the December 6 update.
Edit: Now to get caught up on the mountain of attachments I have sitting here. Intuit, I will be sending you an invoice.
