I have been inside and out of CZ decocker system. The sear return spring leg is what holds the decocker lever in place. If the decocker is not working it needs to be taken apart and reassembled correctly. Assembled correctly it cannot back out.
Have you been in the gun?
This is the sear cage on a SP-01 tactical. The hole at 6 o’clock is where the decocker lever goes in. It slides through the sear cage, at 9 o’clock you can see a slot on the sear cage. In that slot is the spring leg for the spring that returns the sear. This spring leg needs to be lifted up in the slot and the decocker shaft pushed past , let off of the spring and keep pushing the decocker. The sping leg will fall in a groove cut in the decocker shaft, you will hear a snap.