Arista MLAG supports STP for Layer 2 loop detection. In fact, most customers enable STP in their MLAG(s) to ensure no

STP EVPN 4.25.1F All Active Multihoming

Spanning Tree Protocol requires each interface to have a unique port number ranging from 1 through 4095.  Arista STP typically assigns port numbers to port-channel interfaces in the order in which they are configured.

STP EVPN All Active Multihoming EOS 4.29.0F