When a giant green amoeba takes damage it spawns some (random-ish) number of little green amoebas, and the total health of the amoebas spawned by that hit equals the damage that was taken. That's so that if all the little ones make it back to the giant one and reabsorb, it gets healed back to where it was before the damage but no higher.
So the really low health is intentional. The max on them being so high isn't really a bug either, but I guess it does look weird.
If you were seeing full-health green amoebas then that would be a bug (unless you hit a giant one really hard, but I don't think the giant ones have enough health to generate a full-health little one at the current numbers) ; is there a reproducible case of that?