I had exact the same problem, running the SR602 with a CB3S Tuya Chip inside a "mini smart switch" from aliexpress. PIR was cycling on and off forever. OUT pin of the PIR was connected to TX1 (P11). So after changing it to P7 it worked normal. I have no clue what the problem was, checked for hours with impedances, capacities, oscilloscope and what not. Hope that might help anyone.

What I checked:[INDENT]
- Running just the PIR with battery and LED as Output: worked
- Running with impedances and capacities within the CB3S mini smart switch: didnt work
...