When viewing sent emails that have been digitally signed, they do not expand in the message detail window. This is true whether they are part of a conversation or not.
In the screenshot below, when clicking on the message in the message list on the left, the message details appear as on the right. The middle email should be expanded.
The passphrase is remembered until application exit, so it is not as if it is waiting for the passphrase. Besides, it is signed, not encrypted.
Clicking on the message on the right will expand it, but that is done automatically for non-signed emails, so why not for signed ones?
This problem does not present when conversations are disabled.