Hi MansourK,
Apologies for the delay in response.
It appears as though Adobe Sign is reacting out of the norm, which could be down to a technical error. To get this investigated, please reach out to our 24x7x365 Support Team following the steps found in the link below:
How to contact support? | Adobe Sign
To automatically recognize a signature field in a document, you'd either need to use a Text Tag or an Adobe Sign Form Field Template. The Text Tag for a signature field is: {{Sig_es_:signer1:signature}}
I hope this helps!