User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:80.0) Gecko/20100101 Firefox/80.0

Steps to reproduce:

I've configured external OpenGPG to use key on smart card.
I've added key ID to account (!) settings and enabled signing by default in account (!) settings.

My account has 3 identities.

When I write message from default identity, everything works as expected.
When I write message form any of other two identities, I can not sign message, as key is not found.

After that I've added Key ID to each identity separately (same key ID for all of them). It works, but "Add my digital signature by default" checkbox works only for default identity and not for others!

When I set this checkbox, close dialog window and open it again checkbox is unset back.

I could enable signature in message editing window, but default doesn't work for secondary identities.

Actual results:

Thunderbird doesn't apply default account settings for external OpenPGP usage to secondary identities.
Add my digital signature by default setting is not saved for secondary identities at all.

Expected results:

Defaults works for all identities, secondary identities could have separate settings.

Tested with TB 83.0b2 on Linux.
Once checked, for me the 'Add my digital signature by default' setting is kept for the secondary identity.
However, when sending an encrypted message from the secondary identity, in the Write window, the option 'Digitally Sign This Message' is not checked, despite of 'Add my digital signature by default' is set for the identity.
As a result the message is sent without a signature, unless the option 'Digitally Sign This Message' is set manually in the Write window.

Also, when verifying a key on a received message where the "from" address is one of the sender's alternate addresses, TB flags the message as Uncertain Digital Signature.