PGP/MIME subjects from Thunderbird are not decrypted/shown

Describe the bug
The actual subject of a PGP/MIME message (as sent by Thunderbird 78.6) is not decrypted and shown in the message list or message display.

To Reproduce
Steps to reproduce the behavior:

1. Send an encrypted message to yourself in Thunderbird with a descriptive subject.
2. Use K9 to view the message list for that account.
3. Open the encrypted message in K9.
4. Return to the message list and find the encrypted message again.
5. Note that subject fields in all views contain only "...".

Expected behavior
After the message is opened the subject encrypted in the PGP/MIME multipart message should be displayed in both the list and the message itself, similar to what's done for messages sent by K9 itself.

Environment (please complete the following information):

• K-9 Mail version: 5.727 (from F-Droid)
• Android version: 10
• Device: Pixel 2 XL
• Account type: IMAP

Additional context
This may be related to this pull request; a comment there (from two years ago) says it's not on the Play Store yet, but I suspect that's out of date.

Looks like turnabout is fair play, in that Thunderbird can't read subjects encrypted by K9 either.

Reproduce the issue by sending a self-addressed encrypted PGP/MIME message from Thunderbird 78.6, then inspect the Android app's PGP/MIME message handling and subject display paths. Done means the decrypted subject appears in both the message list and message view after opening, as it does for messages sent by K-9 Mail.