I'm experiencing similar behaviour but not always
I already received messages stating that default gateway was back the "main" one but most of the time, when this gateway is seen as faulty, Zentyal switches to the second gateway. Main gateway is is disabled, no more stamped as the default one until I change it manually in Zentyal GUI. Interesting enough: each and every time I change, surprisingly, it works. I mean main gateway is operational and not faulty.
So despite capability to set different weight to each gateway in order to manage priority, it works perhaps well if you do not mind using one or the other gateway but doesn't work that well if your goal is to have one main gateway and one being only "failover" in case of failure on the main one.
Still i didn't try with 2.3