Mail deletes my email signature when I fill in recipient address in the "to" field

I'm not sure I understand what you're asking. Nothing changes in my account or my settings. The only thing that I note (and I'm not sure if this is new or has always been the case) is that the initial blank email (which still has my signature showing) has a field located to the right of the "From" field which is called "Signature"; this "Signature" field shows None with up/down arrows; the only other option is "edit". The field shows None both before and after the signature disappears.

Otherwise, as far as I can tell, the only thing that happens is that when I start typing in the recipient's address in the "To" field and then:

(1) select address from the dropdown, OR

(2) click return so that the address is automatically selected,

the signature disappears from the blank email.

However, if I type in a full address (either because it is a new recipient OR if I don't rely on historical addresses remembered by the system), the signature does not delete.

As an aside, I have a second email account which has no signature associated with emails. To send an email from it, I select the second account in the "From" field using the dropdown. However, as there is no signature on these email blanks, I can't tell if something is happening when I use this email account. Similar to the screenshot above, I can see that the "Signature" field on these blanks also shows None with the up/down arrows and only has one option, which is edit.

Hope this answers your question.

So, when I tried that I still had a problem. However, I tried creating a new signature...seems to have solved the problem. Don't know why, but it's fixed now. Thank you.