After evaluating this issue, there are no plans to address it further or fix it in an upcoming release.  Therefore, it is being closed.  If plans change such that this issue will be fixed in an upcoming release, then the bug can be reopened.

This looks like a bug in httpd.

The failed runs on aarch64 all do read the client's request in a single read() -- the request, headers, and the gzipped body. The "deflate_in_filter()" does the inflate completely in the first run while mod_proxy_fcgi is still in ap_proxy_prefetch_input().
The successful run, on the other hand, reads the request and headers in first read() when in ap_proxy_prefetch_input(), and reads the body later when in dispatch().

The above seems harmless and in both situations the inflate is successful and uncompressed data is sent to the FCGI socket to PHP, however, the failed run is missing the CONTENT_LENGTH environment, which does not get sent because "Content-Length" is removed here:

modules/filters/mod_deflate.c:
1184             rv = apr_brigade_flatten(ctx->bb,
1185                                      ctx->header + ctx->header_len, &len);
1186             if (rv != APR_SUCCESS) {
1187                 return rv;
1188             }
1189             if (len && !ctx->header_len) {
1190                 apr_table_unset(r->headers_in, "Content-Length");
1191                 apr_table_unset(r->headers_in, "Content-MD5");
1192             }
1193             ctx->header_len += len;
1194 
1195         } while (ctx->header_len < sizeof(ctx->header));

And while this header is removed again in both scenarios, the failed run happens before send_environment() is done in fcgi_do_request(). The successful run gets the header removed after send_environment() already done (while in dispatch()):

modules/proxy/mod_proxy_fcgi.c:
 983     /* Step 2: Send Environment via FCGI_PARAMS */
 984     rv = send_environment(conn, r, temp_pool, request_id);
 985     if (rv != APR_SUCCESS) {
 986         ap_log_rerror(APLOG_MARK, APLOG_ERR, rv, r, APLOGNO(01074)
 987                       "Failed writing Environment to %s:", server_portstr);
 988         conn->close = 1;
 989         return HTTP_SERVICE_UNAVAILABLE;
 990     }
 991 
 992     /* Step 3: Read records from the back end server and handle them. */
 993     rv = dispatch(conn, conf, r, temp_pool, request_id,
 994                   &err, &bad_request, &has_responded,
 995                   input_brigade);


Commenting out modules/filters/mod_deflate.c:1190 makes the test pass (it's a test, not a fix).

While I believe sending incorrect (pre-inflate or none) Content-Length with a POST request is a bug, setting "SetEnv proxy-sendcl 1" fixes the test.

https://bz.apache.org/bugzilla/show_bug.cgi?id=57087