Acid type of damage persist in TFTD from beginning. Deep Ones uses it, and some other...
What about knife - is it in this version, that they do piercing damage? Because they must not... I make spears for pearcing and other melee weapons for melee damage.
Btw, i has added new type of damage, but it is laser.
Upd.: Oops, indeed, i make a mistake and knife got piercing damage type. Will fix it soon.
TFTD uses Electric Shock damage instead of Acid
Deep One does an electric attack
See for ex
https://www.ufopaedia.org/index.php/Talk:Damage_Modifiers_(TFTD) Acid is removed and replaced with Electric Shock but in your mod you brought Acid back for Armor rating. But unfortunately no enemy in TFTD deals acid damage.
This is because in OpenXcom extended you can add up to 20 damage types so you added Acid on top of what TFTD already had.
edit fixed the link it was ponting to UFO
Here are the damage types in TFTD
Armor Piercing
Incendiary (Phosphor)
HE High Explosive
Gauss
Sonic
Stun (Freeze)
Drill (Melee)
Electric Shock
You also added Laser and Acid but none of the aliens in TFTD deal Laser or Acid damage or have any form of resistance to those damage types. So this means if you add Laser it will deal 100% damage to all units unless you go and add Laser stats for all their armors
Just a heads up. Whenever you add a new damage type you need to re-balance all the armors in the game including alien resistances. Otherwise laser may produce a kind of imbalance to the whole gameplay. For ex Lobsterman generally are really hard to kill so you need to use melee weapons BUT since they have no Laser or Acid resistances using any of those damage types kills them from range so no need to use melee. The whole point of Drill and melee weapons in TFTD was Lobstermen or weapon durable aliens
Also it seems your Knife should probably deal Armor Piercing damage actually because this would make it less effective against Lobstermen having to use Drill (melee) damage instead. So it would be effective against weaker aliens without tough plating. So even tho was accidental I think it makes sense to be weaker than Drills and diff damage type.