According to the documentation, when no src_mac (or dst_mac) is supplied to the flow declaration, a random one is chosen. However, the "random" MAC addresses do not stay consistent within the flow, but change for each generated request and response.

This is a feature request (or perhaps bug fix depending on how you look at it) to keep, by default, the src and dst MAC addresses consistent within each flow, even if they are chosen at random during session setup.