Since we upgraded our Matomo 4 On-Premise to 5.0, the number of visitors has halved, page views have remained the same and visit duration has doubled up.
We downgraded it to 4, but the wrong measurement remained.
I posted graphs on this topic:
Has anyone experienced the same? 
Similar issue, I use Motomo and a live help platform for tracking visitors, my Motomo is self-hosted, and since going to 5.0 I noticed my traffic had dropped by more than 90% + in the Motomo platform. My live help program shows much more visits, (real users, not bots). The Do Not Track, is not checked, and have tried numerous settings, replace code, etc, still no better results.
Hi @andraspii, @Jeffrey007
I hope @innocraft can give some clues about this strange behavior?
Hello everyone. For us to investigate on this further, could you please send us an email to hello@matomo.org with your system check report and the URL of your website. Please see this guide on how to find the system check report: https://matomo.org/faq/troubleshooting/how-do-i-find-and-copy-the-system-check-in-matomo-on-premise/
@andraspii @Jeffrey007 Hi, we’ve been experiencing the same issue since we upgraded in December. Have you managed to determine what happened? Are you still having problems with the measurement results?
If you are behind a load balancer try adding proxy_ip_read_last_in_list = 0 to config/config.ini.php after the upgrade to Matomo 5.
See: https://github.com/matomo-org/matomo/issues/21793#issuecomment-1956349426
The same at us. We have the half count of visitors and the double time of visiting duration since the change to Matomo 5. 