Why MT give its MAC to a PC arp table.

Hello community, i just got MT RB750GR3 to connect our main wan link and our backup wan link. I should to re-write my post to clarify what was going on.
The IP Phone(GS) connect to jack in the wall(switch), then from the phone a second patchcord goes to PC.
In a plane network(Router->Switch->devices) everything works, slow but works. We decide to install a Mikrotik to manage our 2 wan links and start isolating our devices(NVR, Phones). When i connect PCs without phone to Mikrotik everything works, but when connect IP Phone with PC, only phone get to the Mikrotik. I just check arp table(pc) i didn’t find MT’s mac, manually added L2 address works.

Why PC from IP-Phone didn’t populate its arp table dynamically?