It looks like Salt syndic doesn't push the events to all masters (anymore).
This change was supposed to be implemented in this PR
#43526

I have a salt syndic with multiple minions connected to it, and the syndic connects to two MoM.
When I am running this command from one of the multi-masters:
salt '*' test.ping
I am getting random timeout responses from the minions.

If I set the master_id option in the master.conf on the MoM, then it works correctly.
However, this shouldn't have been necessary, as per PR above.
Tested using 2018.3.4 and 2019.2.0 and I am seeing same behavior.