TRIAC failure modes
In some cases the triac in your circuit can be dagamed typically because of overcurrent, overvoltage or too much heating. Triac failure modes can be either "blown open" or "blown short".
If the Triac blows open, all connections are opened up as the semiconductor material has failed catastrophically and the explosive force inside the device clears disperses the internal wiring.
If the Triac blows short, the shorts can be to any combination of the terminals. MT1 to MT2 (gate open), MT1 to gate (MT2 open), MT2 to gate (MT1 open) or MT1 and MT2 to gate.
There are normally no half-way houses in the failure modes. They go either fully short or fully open. The most common failure mode seems to be a short circuit.
Can you show us a pic of the blown one. I spent some time trying to find a cross for it to no avail. I think it may be a SMD code. I will look some more later.