Not sure if this applies to all mobs/players but at Legion Hold but the mobs Shadow Council Warlock do not break sap when you garrote them, also works with any other rogue DoT but after the DoT finishes it breaks sap.
SS:
At start:
And at end:
You can see the mob still sapped with garrote ticking.
Hopefully you'll be able to "get it" now.