Choose signature drop-down menu in Mac Mail is greyed out

This is a work around to an ongoing issue (or just confusion about how the very non-intuitive Mail signatures work):

I was setting up several email accounts on a "new" Mac (2020 13" MacBook Pro 4TB).

In Mail | Settings | Signatures, all previous signatures I've used are listed under All Signatures.

But for each account (initially showing 0 signatures), the Choose Signature drop-down box is greyed out.

So there seems to be no way to re-use a signature from that All Signatures list - it seems you have create a new signature for each account all over again (and yes, I selected the mail account in the left pane first).

The workaround is simply to first select the All Signatures list, then from the list select and drag the signature you want onto the Mail account you want to assign it to.