Steps to reproduce:

1) install Ubuntu 22.04 LTS.
2) update System
2.1) sudo su
2.2) apt-get update && apt-get dist-upgrade -y
3) install Apache, PHP-FPM and Dependencies 
3.1) add-apt-repository ppa:ondrej/php
3.2) apt-get update && apt-get install apache2 libapache2-mod-fcgid php7.4-fpm brotli -y
4) Configure PHP-FPM
4.1) useradd default
4.2) mkdir /var/www/alternative/ ; chown default. /var/www/alternative/
4.3) /etc/php/7.4/fpm/pool.d/default.conf

[default]
user = default
group = default

listen = /run/php/php7.4.sock
listen.mode = 0660
listen.owner = www-data
listen.group = www-data

pm = ondemand
pm.max_children = 15

4.4) service php7.4-fpm restart
5) configure Apache
5.1) a2enmod brotli proxy proxy_fcgi
5.2) /etc/apache2/apache2.conf

...
<Directory /var/www/>
	Options Indexes FollowSymLinks
	AllowOverride All
	Require all granted
</Directory>
...

5.3) /etc/apache2/sites-enabled/000-default.conf

...
DocumentRoot /var/www/alternative/
SetHandler "proxy:unix:/run/php/php7.4.sock|fcgi://localhost"
...

5.4) /etc/apache2/conf-enabled/z-server.conf

LogLevel notice

5.5) service apache2 restart
6) create PHP script
6.1) /var/www/html/.htaccess
SetEnv no-gzip 1
AddOutputFilterByType BROTLI_COMPRESS text/html

6.2) /var/www/html/.user.ini
output_buffering = Off

6.3) /var/www/html/seg.php
<?php
ini_set('display_errors', 1);
for($i=0;$i<500;$i++)
        f;

?>

7) curl -vvv --compressed -H 'Accept-Encoding: br' http://127.0.0.1/seg.php


----------------


directly connected to php-fpm over cgi-fcgi gives the expected 500 "Warning Messages" whereas curl (uncompressed) only giving 333 without any logged error and/or segmentation fault:

#install package for cgi-fcgi
apt-get install libfcgi-bin

export REQUEST_METHOD=GET;export SCRIPT_NAME=/var/www/alternative/seg.php;export SCRIPT_FILENAME=/var/www/alternative/seg.php;cgi-fcgi -bind -connect /run/php/php7.4.sock 2>/dev/null | grep "Warning" -c
#gives: 500

curl http://127.0.0.1/seg.php | grep "Warning" -c
#gives: 333

cat /var/log/apache2/error.log | grep 00:00:00 (time of error) | grep Warning -c
#gives: 500