They only heal specific type of damage, to make distinction which wounds are healed by what get some custom client like amber or follow wiki.
Leeches deal HHP damage called "Leech burns", if they heal any wound type you might have currently the "Leech burn" will replace it. So let's say you have Blunt Trauma 18, you then get Leech Burn from q10 leeches and it will look like this:
Blunt Trauma 15
Leech burns 3
wiki
hereamber
hereThe way you describe it worked in legacy.