Premature end of script headers due to response header length

Question

My question is very similar to this unanswered question: Apache 2.4 + PHP-FPM + Chromelogger (how to increase HTTP header length limit?)

Initially I was getting:

[proxy_fcgi:error] Premature end of script headers

Then after disabling the proxy:

[cgi:error] Premature end of script headers: ea-php74

I am sending debugging data in the response headers and these errors are generated any time I send response headers around 8K or more. Seems somewhere along the way the headers are being truncated.

I've read about adjusting LimitRequestFieldSize to no avail as this setting limits only request header length, not response headers.

Is this due to PHP or Apache? Is there a setting to limit the maximum allowed response header size?