inexplicable field string_city_processor_string

An inexplicable field with name="contact[string_city_processor_string]" appeared, how to remove it (how can this be debugged)?

It appears at the end of any form template anyway, here is the full html of the field:

<div class="form-group">
    <label class="control-label" for="contact_string_city_processor_string">String city processor string</label> 
    <input type="text" id="contact_string_city_processor_string" name="contact[string_city_processor_string]" tabindex="-1" autocomplete="off" class="form-control">  
</div>

here is one of my form-config file:

notification:
    enabled: true
    debug: true
    debug_smtp: true
    debug_address: admin@some.com
    subject: "Message from site"
    subject_prefix: ""
    replyto_name: name # Email addresses and names can be either the
    replyto_email: email # name of a field below or valid text.
    to_name: Sale
    to_email: sale@some.com
    from_name: name
    from_email: email
feedback:
    success: <b>Thank you.</b><br>Your request has been received.
    error: There are errors in the form, please fix before trying to resubmit
submission:
    ajax: true
database:
    contenttype:
        name: contact_form # save all form submissions to the quotations contenttype
        field_map:
            # timestamp: ~ # do not save the timestamp
            # url: ~ # do not save the url
            # path: ~ # do not save the path
            # ip: ~ # do not save the ip
        ignore_missing: true # ignore fields in the form that aren't defined in the ContentType
templates:
    form: forms/contact/_contact.twig
fields:
    name:
        type: text
        options:
            required: true
            label: Name
            attr:
                placeholder: Name
            constraints: [NotBlank, { Length: { "min": 3, "max": 128 } }]
    company:
        type: text
        options:
            required: true
            label: Company
            attr:
                placeholder: Company
            constraints: [NotBlank, { Length: { "min": 3, "max": 128 } }]
    email:
        type: email
        options:
            required: true
            label: E-mail
            attr:
                placeholder: E-mail
            constraints: [NotBlank, { Length: { "min": 3, "max": 128 } }]
    phone:
        type: text
        options:
            required: false
            label: Phone
            attr:
                placeholder: Phone
            constraints: [NotBlank, { Length: { "min": 3, "max": 12 } }]
    comment:
        type: textarea
        options:
            required: false
            label: Message
            attr:
                placeholder: Message
    subscription:
        type: checkbox
        options:
            required: false
            label: Upgrade your skills by subscribing to our monthly newsletter
            label_html: true
    policy:
        type: checkbox
        options:
            required: true
            label: I've read the <a target='_blank' style='color:\#fb9024;' href='/privacy-policy/'>Privacy Policy</a> and I consent to the processing of my personal data.
            label_html: true
    submit:
        type: submit
        options:
            label: Get in touch
            attr:
                class: button _submit _w-full _size-l

bolt / forms

inexplicable field string_city_processor_string #123