Getting almost half of the actual upload speed while using WAX206 (WIFI6)

What comes into my mind reflecting the tech specs of the two WAX206 Wi-Fi  radios

...

- Up to 800 Mbps (11ng, HT40, 256-QAM): Performance mode. This mode
allows 802.11n, 802.11g, and 802.11b devices to join the network and allows
802.11ng devices to function at their maximum speed. This mode is the default
mode.

This mode supports a 40 MHz–wide channel and 256 QAM.
• 5 GHz mode: Select one of the following WiFi throughput modes for the 5 GHz
radio:

...

- Upto 2400 Mbps (80 MHz)(11ax, HT80, 1024-QAM): Performance mode.
This mode allows 802.11ax, 802.11ac, 802.11n, and 802.11a devices to join
the network and allows 802.11ax and 802.11ac devices to function at up to
2400 Mbps. This mode is the default mode.
This mode supports a 80 MHz–wide channel and 1024 QAM. 

One possible reason why the 2x2 Wi-Fi clients on the mobile devices tested are pure 2x2 devices, while the WAX206 does employ 4x4. This does divide the possible throughput in two, resp. leave a second half of the available capacity fir a second session.

Nothing wrong with the on-board speed test of the WAX206 - this does not make use of the wireless path, it's direct wired LAN.