I don’t know who needs to hear this, but “Yes, MLAG requires STP!”
I just finished pulling my hair out for a few hours with MLAG peers not staying in sync - the chief symptom being 4% packet loss across a tagged peerlink. RouterOS 7.15 implemented logging that reports MLAG peer events, and this was key in confirming that there was a problem.
This post by @robertpenz is a great list of must-do’s for MLAG - http://forum.mikrotik.com/t/mlag-issue-mlag-functionality-flaps-lacp-system-id-of-secondary-when-primary-reboots/157426/1