I currently have this setup:

RB951G-2HnD (6.43.7 (stable)) in switch mode, wifi in station-bridge connected to
cAP ac (6.43.4 (stable)) in switch mode, wifi in ap bridge connected by LAN to
hAP ac^2 (6.43.4 (stable)) in router mode, acting as dhcp server

I have a device connected by lan to RB951G-2HnD, the hAP ac^2 see the dhcp request and provide it with a proper IP

is it normal that the dhcp lease for that device use and show the proper MAC of that device but the ARP list from hAP ac^2 is showing the MAC of RB951G-2HnD instead of that device?

It happens when you use repeaters, and you made one.