Opto-Isolater always on by higher voltage
The AQH3213 is an optically isolated triac device for controlling AC loads. It should not be used on DC loads because the triac, once properly "fired" will remain on all the time until power is removed. 

The reason it "appears" to work correctly at the lower DC voltage is that the triac will act as an on-off device if the current taken by load is below the threshold-latching level. Once above the threshold level it will remain permanently activated unless power is removed or cycled (as when used in an AC application).