Encrypted subject stays clear once encrypted

Hi, today I tried the first time encryption of subject lines. I did it by sending an encrypted mail to myself. When I received it then I see that subject is encrypted - good. When I open the mail I need to enter my key’s passphrase, subject line and mail body are decrypted - good.
When I move back to mail list I see that subject line is still in clear text. Mail body is shown as encrypted.

Is this expected behavior? I did not find an option to keep subject line encrypted as long as the passphrase is not stored in memory

Yes, this is intended behavior. The decrypted subject is written to K-9 Mail’s message database.