Is it AX50 really got hardware problem?
I just bought 1 new AX50 to replace C1200

I put the router on 2nd floor as previous c1200 place
I am using unifi 100mbps

WiFi speed at same 2nd floor still maintained the same speed
2.4ghz can up to 70-90mbps
5ghz can max at 100mbps

But ground floor WiFi 5ghz speed worse than c1200
Only can get 10-30mbps (c1200 can get 80-100mbps)
Surprisingly the 2.4ghz speed improved with ax50 at ground floor can get 50mbps (c1200 only get 20-30mbps)

Is it ax50 wifi 5ghz wall penetration weaker?
I expect it should perform better than c1200