The bug:
Blocks are faster destroyed or replaced by fire in humid biomes. Expected would be the opposite.

Code analysis:
Based on 1.12 decompiled using MCP 9.40 PRE 1

The method net.minecraft.block.BlockFire.updateTick(World, BlockPos, IBlockState, Random) subtracts 50 from the value which is passed to the method net.minecraft.block.BlockFire.catchOnFire(World, BlockPos, int, Random, int) as chance if the fire block is in a humid biome (such as a jungle). The condition for fire to destroy or replace the block is random.nextInt(chance) < i, which means the smaller the chance value the likelier it is that the condition is true. Therefore the method BlockFire.updateTick should probably add 50 to the value instead.