A couple additional notes:
After a FW upgrade Zooz recommends resetting the switch using the tap-tap-tap&hold for 10 seconds the upper paddle until the blue light blinks.
The Zen26 and Zen27s use a FET for the output stage. Of course I don’t know the internal design but it makes them more susceptible to this type of behaviour.
I understand you didn’t have any issue with previous FW versions but with only two dimmable leds as a load I am highly confident that it is now the cause of your issue.
If you could stand replacing your lights temporarily with incandescent bulbs I think it would solve your issues and prove the point.
I could also send you a couple load resistor to try out.
