It seems that first aid kits use a simple setDamage command to "heal" the player. What it means is that all individual body parts (or hit points), even those that originally had less than 0.25 damage, will have their damage set to 0.25 due to the first aid kit's all-encompassing setDamage.

Obviously, body parts that don't have more than 0.25 damage should be unaffected. The best way to go about it would be to check each hit point individually and setHitPointDamage if needed (or use the command to retain the original damage). Ideally, the game should feature a command to only set the main damage of a unit, not affecting hit points at all.