Cannot update Tag via API

Cannot update Tag via API

I'm trying to change Tag via API, but it has no effect.
I've tested my code - it can update other fields, so I suppose something is wrong with the POST body.

Here is my POST body sent to
/crm/private/xml/CustomModule1/updateRecords?newFormat=1&version=4&scope=crmapi&authtoken=****

  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>
    <CustomModule1>
      <row no="1">
        <FL val="Tag">tag2</FL>
        <FL val="Id">2117421000000627709</FL>
      </row>
    </CustomModule1>