Why is a post/upload of a 100k JSON file to an AWS CloudFront endpoint resulting in 504 error?

On a wordpress site, we have requests coming in to CloudFront, which sends the request to origin (if necessary) which goes through an ELB, and to two or three instances that will service the request.

Most requests work, but when we upload a JSON file to admin-ajax.php, it results in a 504 error that we captured from the CloudFront logs:

2022-01-31  21:32:24    MIA3-C2 1462    67.190.247.197  POST    d2q8ixmwt0jy43.cloudfront.net   /wp-admin/admin-ajax.php    504 https://xxxxxxx.com/wp-admin/edit.php?post_type=elementor_library&tabs_group=library    Mozilla/5.0%20(Windows%20NT%2010.0;%20Win64;%20x64)%20AppleWebKit/537.36%20(KHTML,%20like%20Gecko)%20Chrome/97.0.4692.99%20Safari/537.36%20Edg/97.0.1072.76 -   -   Error   Ae0gLzThCiZR2N5D8gLO6s-o8IwwlmYrxWvUbUgr1A64_nMpzN0qRg==    evolvetelemed.com   https   167737  30.107  -   TLSv1.3 TLS_AES_128_GCM_SHA256  Error   HTTP/2.0    -   -   64798   30.107  OriginCommError text/html   1033    -   -

The error appears to be an OriginCommError, but I cannot see why it is getting that error. Tailing the logs on our web servers on the instances shows that the request is not even reaching there.

I am not exactly sure why some requests would work and some wouldn't. To add further conclusion, when binary files in the media library are uploaded to async-upload.php, it works.

We use nginx as a webserver on the front end using php-fpm for processing php.