You sure it wasn't a passive monk heal or something after you finished the spell?
no the healing of ST stopped only the damage kept on
edit: tried it a few times in a 1v1 ai match as pacov suggested but i wasnt able to reconstruct the scenario. however im 100% sure it did happen in some of my other matches (a pity i dont have a replay). that white number which tells you when you do damage and how much damage you do still pops up after you cancel ST and the tower takes the corresponding damage.