Strange Mac adress after connecting Openwrt

I'd like to know how to get rid of this.
I have a nas that was connected diretly through lan cable to my non-openwrt 4G router.

Router => NAS

To have wifi in the area, i put an open wrt router configured as dumb AP on the cable and plugged the router in it.
So now i have
Router =>OpenWRT DumbAP=> NAS

The router was configured to set a fixed IP on the NAS through Mac Adress but i noticed that now the nas mac adress that I see in the router is changed to a weird ‏00:10:20:30:40:50.

Why that?
Another device connected with wifi has a normal mac adress...