According to the WPForms team, You will need to contact your web host. In every case so far it's been a security rule (ModSecurity or similar) or server configuration that causes these issues.
The issue is coming from the web server app on your host prior to WPForms getting this request. You should reach out to your web host for assistance in increasing the URI limit that your web server is willing to forward along to the applications hosted on said host.
If you're not seeing data from all of the fields that have been filled in (or even seeing that the field exists), please check to see if you have any add-ons for WPForms. One plugin that was found to conflict (apparently discontinued on March 19, 2019) is Fancy Fields For WPForms Lite Plugin Including File Upload. When used in conjunction with the upgraded version of WPForms, it caused certain fields to not make it to Zapier.