You're overpowering your Subtank by the sounds of it. Seems to be the max wattage for a Subtank is about 25 watts. You're applying around 30 watts at .5 ohms with a mech.
Yep. I find that [especially with a high-VG juice] the Subtank coils (the OCC in particular) will straight-up BURN at 25+ watts.
At 25W through a .5 ohm coil, you're only asking for an easily-attainable 3.5V (on a regulated mod you can easily set it to this voltage). A fully charged (18650) battery in a mech mod, however, will dump out 4.2V. As the battery discharges, the voltage will get lower. At 4.2V through a .5 ohm coil, it's hitting 35W, which is 10W beyond the "you might burn your head" range. 35W almost GUARANTEES you're gonna burn your OCC. The testing it took for me to figure this out was.. Expensive. I figure the main reason for this is the Kanger heads don't wick as effectively as, say, an RTA does. I can push 60W through my .6 ohm Orchid without burning anything. Granted, the coils are built VERY differently when I do them versus in the Kanger heads.
Anyway, when you use a 1.2 ohm head with a fully charged mech battery, you're hitting 14W. When I use my regulated device and 1.2 ohm heads, I find myself having to crank my voltage up to about 4V. I haven't burnt any 1.2 ohm heads yet, I've noticed it's a lot easier to burn out the .5 ohm heads (but that depends on what wattage/voltage you're trying to hit it at).
That's a bit wordy, but that's a pretty thorough explanation for why this is happening. My best suggestion would be to get a regulated device that can handle .5 ohms. Being able to fine-tune your vape according to your attached device is a SERIOUS benefit.