A triac coupler is a semiconductor device; even when it is OFF, leakage current IDRM occurs, and especially at a high temperature, a larger leakage current occurs. Whether or not this can cause malfunction to a device or load at the second-half state must be checked. Also, even if it is equal to or less than the maximum rating, if a suddenly rising voltage (dV/dt) is applied, malfunction may occur. Care should be taken.
Refer to Toshiba Application Note "Triac couplers-basic properties and application design", sections 10.7 to 10.9.