I have had this behaviour on zen21, zen71, zen26, zen76, and zen27.
The zen21 and zen71 were my fault - I am using them on bathroom exhaust fans. A snubber cct on the output fixes it.
The others, a load resistor (or a bypass) fixes it.
Possible not the only way to fix it but switches are sensitive to noise from the load. Also when they are off, there are considerable peaks on the output. Only an oscilloscope will show you what you are dealing with.
My default for new installations is a 24k load resistor in parallel with the output.
