There are some "3/4-levels" domains on the NTA Online Demo such as login.oscar.aol.com or email.corp.hdq so I would first think this is a configuration issue in your environment, but I am not sure.
When you perform a reverse lookup on the ip addresses that belong to the companyname.eu.com from the Orion/NTA server, do you only get the machinename.eu.com resolution or the one expected? If this the case then the DNS servers have a configuration problem or they are not configured properly on the Orion/NTA server.
NTA logs are stored in the \Program Files\SolarWinds\Orion\NetFlowTrafficAnalysis\swdebug.log.X files .
They might contain errors about the DNS resolution of these endpoints.
If not it is possible increase the logging level of the NTA module \Program Files\SolarWinds\Orion\LogAdjuster.exe
I suggest you open a support ticket and attach the diagnostics once received the email confirmation.