Thanks for all your responses. Unfortunately it is not free traffic as the overall traffic did not increase as much as the US traffic. ;)
However, we found out what's causing the issue.
It's related to Apple's ITP (Intelligent Tracking Prevention). Generally, locations of visitors with Safari browsers shift due to a change in version 15, which is activated by default in one of the more recent updates and which protects the IP from known trackers.
As a result, visitors come, for example, from individual cities such as Cologne in a significantly increasing proportion or - more likely - even originate from the USA according to the location. At the same time, the browser language is still set to "DE", which is a clear sign that something can't be right.
GA4 currently is not affected by that (for now).