Maximum Form Fields... is there a restriction?

I have a preexisting admin form (from 2010) that has ~900 form fields and performs a POST request using enctype="multipart/form-data". Normally this isn’t an issue. We can perform the request against the origin server directly and both the web server & web application handle it quickly and without any issues. (I also don’t recall having any issues when using StackPath or Edgio WAF services.)

I’ve noticed that Fastly fails when requests that have 361+ form fields and the encode type is multipart/form-data. Is this a known issue?

The user receives an “IO Error” or “Internal Server Error” message in their browser, but there’s nothing in our origin server log files or anything in the Fastly Signal Science dashboard or in the streaming logs. I only see that GET requests are logged and the failed/blocked/rejected POST are nowhere to be found.

I plan on creating an HTML+JS SSCCE to demonstrate this, but wondered if this was known and if there’s any documentation regarding it. I have a feeling that we may have some other updates to perform as our company has been around since '97 and some scripts may still have some technical debt.

I created a generic test HTML script that geenrates multiple hidden form fields on-the-fly to be submitted. While testing this script today on our of our WAF-protected sites, I could submit 402 fields, but not 403.

QUESTION: Is there any configuration option to increase this or is this a limitation? Where can I find documentation?

NOTE: In this specific case, I ended up writing a client-side & server-side work-around using javascript to convert eligible fields to JSON, append a extra textarea field with the JSON payload, disable all of the other fields during submission and then used a server-side script to deserialize to recreate the form parameters.

Here’s the client-side script.