I assume it's a bug, so i've post it first in the support forum.
but i will post it there too so.
all is in the title. stacking boat with allies parially denied your torpedo and the torpedoes of your allies. it result into 3 destro having same dps of 1 destro.
in the replay :
anti torpedo work on allies unit.
from 11 min 50 to 13 min.
look at pink and orange destros.