I’m still in the early stages of playing with this now known “loophole” and just realized an off-setting effect…
You can move a sub into a convoy zone with the enemy fleet and force him to choose between attacking the sub or making noncombat moves freely and leaving the sub alone, but even after leaving the sub alone for a turn, if the fleet remains there, then the sub is forced to leave on the next turn to avoid combat (assuming 1 or more destroyers there). And wherever it moves to, it is in range of the enemy fleet.
The sub can’t just sit there turn after turn if there is an enemy destroyer there - it only works once. Kind of blunts the effect.