Ancient debris generates exposed to lava and fire

1. locate a lava lake under a nether waste biome (doesn't work on other biomes due to the bug MCPE-74607 )
2. run a fill command that replaces near lava with air (/fill ~20 32 ~20 ~-20 18 ~-20 air 0 replace lava)
3. look for debrish

soon after (it may take some fill comands) you will find deprish exposed to air (previous lava from the lake)

you shouldn't find debrish as the debrish should not be exposed to air or lava

About fire, its random if it generates since fire is less common than lava in lava lakes, but it happened once as you can see on the photo.

 

has been resolved!