Hi,
i have a similar issue, back in the days i bought a bunch of MikroTik rb951ui-2nd and i was quite happy with them. Then i needed some more bandwidth so i decided to replace them with a bunch of C53UiG+5HPaxD2HPaxD. I realized that some clients are not connected, after some searching i found this thread and this one: http://forum.mikrotik.com/t/hap-ax3-wireless-problem/172468/1 For me this sounds like a major issue with those C53UiG+5HPaxD2HPaxD. Is there any solution available or still in research?
I’m not sure where to respond, if it should be here or on this http://forum.mikrotik.com/t/hap-ax3-wireless-problem/172468/1 thread.
I have also tried some different antennas i had lying around, with no luck. Maybe i can help with providing some logs or data? Please let me know, but i’m not a pro on this. Be kind 
Best regards,
snooops