2 Tile Mob Entities | Weird Collision of Lava Source/Blade

So I was messing around in 1.13.1 testing out some architectural builds, redstone, new mechanics features that I do when testing out new updates.

Then I gotten myself into making a👎 auto-mob farm killing chamber using the lava source block or the lava blade to kill the mobs, getting their drops.

But then this gets strange; When a 2-tile mob entities (excluding creeper, it's heighth is 1.7 blocks) when standing on slabs, they take fire tick damage, giving it the higher priority over the lava source block/blade burning affect. I even checked the F3+B of all 2-tile tall Mobs hit-box and done this in nighttime, they're heads are definitely touching the lava source/blade but are taking fire damage than the lava burn damage.

Is this some inconsistency with fire tick damage with 2-tile tall mobs since the mobs themselves are standing in lava, when lava burning tick damage isn't affected? I just wanted to make sure if it is but, if it isn't if, I may have made a false bug report don't punish for my knowledge if I don't check with Minecraft Gamepedia website.