With matomo 4.5 I get too low numbers


since the update to matomo 4.5 we get too low numbers for visits. The numbers we get are about half of what really happens. Before the update visits where consistent and slowly growing, so my guess is, they were correct.

single page
nginx: 1.14.2-2+deb10u4
matomo 4.5:
PHP: php-fpm 2:7.3+69
mariadb: 1:10.3.31-0

Logs are not helpful, diagnostics:run neither.
We are totally clueless. Could php-fpm be the culprit?

Thanks for reading!