In case of Nokia 8, It was supposed to be one of the first smart phone running Android Oreo during the launch. Unfortunately it was not the case and surely it got first preference for Oreo Beta testing since it was a recent launched phone and a flagship phone for Nokia.
Can't say the same unless we have the actual sales figure for Nokia 5 and Nokia 6.
Nokia 6 was online only model in India, so it could have done better sales than Nokia 5 which was offline exclusive model as it had a marginally better configuration.
Nokia 6 Oreo Beta out!