Steps to reproduce:

1. Download the demo world MCPE-79960 demo.mcworld and open it. Note: There are two nearly identical copies of a partial automatic chicken farm/cooker. The only difference between them is what kind of block is behind the lava blade.
2. On each farm in turn, toggle the lever to activate the egg dispenser. After two or more chicks have hatched, toggle the lever off again.
3. Feed seeds to one of the chicks until it turns into an adult chicken.

Expected results:
In both farms, the adult chicken gets cooked by the lava, and the remaining chicks are left alone.

Actual results:
The left farm works as expected, while in the right one the remaining chicks are burned up along with the new adult.

There's no easy workaround for this problem, but you can modify the farm to use a different design that doesn't trigger the bug. The block above the dispenser has to be a full-size solid block. It's probably a hopper at the moment, so you'll need to move that hopper to the back or one side of the dispenser. That will set your egg-layer chickens free, unfortunately. but you would need to move them anyway to be above the new hopper position. You may wish to look up a tutorial on YouTube that uses this alternative design.