The STm's are a solid tank, I use 2 exclusively and I never have any leaking, though I also use the RBA V2 in both of mine. I have used the OCC heads in the past, still never had any leaks, but historically pre-built coils can be very hit and miss.
What wattage are you
vaping at and what air airflow setting are you using? Those settings should also be adjusted based on the OCC you are using. It is possible you are drawing too much liquid into the coil and not burning it off fast enough, if you are on a tight AF and low watts.
Like I said, I use the RBA and build my coils at 1.2Ω with my AF set on the smallest setting with the mod set to 20.5 watts. Perfect vape for me, never leaks, even when it has been on it's side for an extended amount of time (30+ minutes).