Why does Bitrix sometimes not process FOS?

Question

1C-Bitrix

The crux of the matter is this. The site has a regular form. Phone and name. The form is made by the bitrix:form component. The essence of the problem is that sometimes the form does not work out as it should. The fields are cleared, but there is no submission message. As well as the dispatch itself. The message does not reach the admin panel at all, since it is not in the b_event table, there is no information about its sending in the SMTP log of the module. Well, actually it is not added to the infoblock.
In the course of search of a problem it was found out that. Below are two examples of what we see in the developer console in the network tab. ajax forms
Form NOT sent

------WebKitFormBoundaryLTYlAppKKcGVPD5Y
Content-Disposition: form-data; name="bxajaxid"
fd5071e2c8b2fe2b5d506ef4ea44aaf1
------WebKitFormBoundaryLTYlAppKKcGVPD5Y
Content-Disposition: form-data; name="AJAX_CALL"
Y
------WebKitFormBoundaryLTYlAppKKcGVPD5Y
Content-Disposition: form-data; name="sessid"
c24819b141870334d6e7ecaa66684496
------WebKitFormBoundaryLTYlAppKKcGVPD5Y
Content-Disposition: form-data; name="WEB_FORM_ID"
6
------WebKitFormBoundaryLTYlAppKKcGVPD5Y
Content-Disposition: form-data; name="sessid"
c24819b141870334d6e7ecaa66684496
------WebKitFormBoundaryLTYlAppKKcGVPD5Y
Content-Disposition: form-data; name="form_text_27"
test
------WebKitFormBoundaryLTYlAppKKcGVPD5Y
Content-Disposition: form-data; name="form_text_28"
[phone]
------WebKitFormBoundaryLTYlAppKKcGVPD5Y--

The form was submitted successfully.

------WebKitFormBoundarycCXRrW7ewOXQ9xHd
Content-Disposition: form-data; name="bxajaxid"
fd5071e2c8b2fe2b5d506ef4ea44aaf1
------WebKitFormBoundarycCXRrW7ewOXQ9xHd
Content-Disposition: form-data; name="AJAX_CALL"
Y
------WebKitFormBoundarycCXRrW7ewOXQ9xHd
Content-Disposition: form-data; name="sessid"
c24819b141870334d6e7ecaa66684496
------WebKitFormBoundarycCXRrW7ewOXQ9xHd
Content-Disposition: form-data; name="WEB_FORM_ID"
6
------WebKitFormBoundarycCXRrW7ewOXQ9xHd
Content-Disposition: form-data; name="sessid"
c24819b141870334d6e7ecaa66684496
------WebKitFormBoundarycCXRrW7ewOXQ9xHd
Content-Disposition: form-data; name="form_text_27"
test
------WebKitFormBoundarycCXRrW7ewOXQ9xHd
Content-Disposition: form-data; name="form_text_28"
[phone]
------WebKitFormBoundarycCXRrW7ewOXQ9xHd
Content-Disposition: form-data; name="web_form_submit"
submit
------WebKitFormBoundarycCXRrW7ewOXQ9xHd--

Plus to this. Upon successful submission, we get the status code 302 on this page, and if it fails, we are left with the code 200.
Actually, the difference in request is only in a certain submit parameter. I don’t know Bitrix internals so well to understand where and how this request is formed, and therefore I don’t understand where to dig to solve the problem.