Don't benchmark WAN over Wi-Fi. Use Ethernet. The radios in the client devices have such staggeringly different quality between manufacturers it's almost a joke to rely on any of their advertised specs.
Once you have your baseline then start troubleshooting Wi-Fi. I suspect it may be but think it's too early to call any sort of 'bug'. You'll want to bench the Wi-Fi to the AP, not the WAN, first. See the below script to locally hosting OpenSpeedTest if you don't care for iperf3.