debian package zabbix-frontend-php do not depend on packages:
php7.0-mbstring, php7.0-bcmath
And without these packages I see such error messages in apache log:

PHP Fatal error: Uncaught Error: Call to undefined function mb_strlen() in /usr/share/zabbix/include/audit.inc.php:104

PHP Fatal error: Uncaught Error: Call to undefined function bcadd() in /usr/share/zabbix/include/classes/db/DB.php:117

Also package php7.0-xml is not strictly necessary, but desirable, So it should be in Recommended or Suggested sections of dependencies.

BTW: official ubuntu package zabbix-frontend-php (version 2.4.7) do not have such dependency problems.