Why does the page break when concatenating php strings?

Question

PHP

Your server.
apache
Red hat linux
php -
fpm
5.6

a data set.

I know that Bitrix uses a buffer before output, so the question is "why does the text output to the page stop, or rather the string is not further concatenated? The script itself continues to work, checked by logging to the file"
php-errors.log is empty, in ini is set, in phpinfo the option is visible to the set
apache error_log is also empty there are
rights to write to the log