Do you always get the same results for www.amazon.de when you ask Google DNS directly?Just an idea: I assume you do not get the same results from Google DNS, when you repeat your query later. Thus asking your OPNsense DNS might provide slightly different results, depending on its own cache.
that was also my assumption that it must have something to do with the cache. I was just wondering because I no longer had access to the amazon pages until I set the google DNS servers directly on the hosts (instead the OPNSense IP)
You probably would have mentioned this, but it's still worth asking: Do you have any overrides configured in your Unbound DNS?