@privateflyer05
Let me explain in details:
The issue you're describing looks similar to the one I posted, but the device used is different - Archer C6 v2.
That issue is valid for both cabled/wirelessly connected devices.
The OP in the thread in question mitigated the problem by setting the device's DHCP server from OFF to Auto:

If the AX21 doesn't have this option when configured in AP mode then that won't work for you. That's why I posted " (if it's applicable in AX21 WebGUI) ".
In general when in AP mode the device's DHCP server should be OFF - here's the TP-Link official guide for configuring wireless router in AP mode (Case 1), using its WAN port to connect to you main router.
Now on:
"I sure hope this issue can be escalated in your engineering group, sounds like you need a different mechanism to identify attached clients ... "
What you can do further if you want to contact TP-Link tech support is to e-mail them using this form and they should reply regarding your issue.