I have this exact same problem. I am using FW RUTX_R_00.02.05.1
As the OP said, if a failover event subtype is selected, alert emails fail without a log entry. However 'Test Emails' work correctly.
If no failover event subtype is selected, alert emails work.
The subtype is important for clarifying whether Main or Mobile has triggered the Failover event.
I have two RUTX11's and they both have the same bug. Please recreate yourselves and fix this Teltonika, these Routers rock otherwise! :)