@quarky
With your setup you cannot trigger the bug.
As I understand iPad is connected via WiFi, right? The WiFi is not affected by this bug, only LAN ports that have simultaneous WAN and LAN traffic going through them with at least one 100Mbps device connected to any LAN port but also transferring data over LAN.
First you run iper3 server on a PC (1) as a LAN server connected at 1Gbps. Then run iperf3 as a client with -R (reverse option) on a client device (2) connected at 100Mbps (that's what triggers it). It doesn't matter if the connection of device 2 is directly to a router LAN port or to the same port via second switch (Gbit or not). Finally run WAN speedtest on a PC (1) connected at 1Gbps. The PC (1) WAN down/up speed drops significantly.
The best scenario is when you have third (3) and forth (4) clients. Both (3 and 4) should be 100Mbps clients.
I run network traffic from PC server (1) to all three (2, 3 and 4) 100Mbps clients. Total around 300Mbps going from the PC (1) to clients 2, 3 and 4 at the same time.
And when I run WAN speed test on PC (1) I have ridiculous WAN performance varying from 20-30Mbps to maximum 40-50Mbps with ping increasing to 50-500 ms (normally 1ms).
With this setup If a client device (2) is connected at 1Gbps to LAN port 2 (I use a Laptop and just change the cable with 100Mbps one) and it doesn't send any data to any other 100Mbps device over LAN then the Laptop (2) can download/upload from/to WAN at full Gigabit speeds. Even if the PC (1) cannot do this because it sends data to any other 100Mbps device.
Obviously the router LAN port, through which the LAN data is sent to 100Mbps device connected to another port, cannot receive/send data at Gbps FD speeds to WAN in this case.
In the network setup above if all devices 1, 2, 3 and 4 are connected at 1Gbps the WAN and LAN performance are great. Even if the PC server (1) sends data at 930Mbps through the LAN to 2, 3 and 4 (now all are connected at 1Gbps) at the same time it's still possible to download from WAN on a PC (1) at 930Mbps. We have expected Gigabit Full Duplex performance - 930Mbps download (with 1 ms ping!) from WAN on PC (1) and simultaneous data sending over LAN from PC (1) at 930Mbps to other LAN clients.
In original Netgear firmware (Voxel's firmware too) this bug is even more difficult to notice because if only one 100Mbps client is present and there is LAN traffic as per the above network setup the PC (1) still can achieve 900Mbps download from WAN at much higher ping though. The upload speed to WAN is affected and noticeable - in my case I have 700Mbps upload from ISP but I can only get something between 100-200 Mbps.
With the stock firmware the LAN performance is affected more negatively and is noticeable. The LAN traffic between PC (1) and 100Mbps client is interrupted during the WAN speed test. This cannot be considered as more desirable defect because the LAN data transfer is interrupted.
If I perform the tests with 4 devices (one 1Gbps and three 100Mbps) the stock firmware begins to suffer from the same WAN download slowdowns but they are somehow biased differently toward the WAN or LAN performance degradation with alternating WAN or LAN performance loss.