Direct Warps don't seem to work unless Link is snapped to the 8x8 grid, and triggers don't trigger if you're walking on the top half of a trigger tile, and the rest of Link is on the bottom half of an adjacent trigger tile.
This is causing some problems for me, seeing as I'm using direct warps to make a person speak when you walk up to them, and make the string disappear when you walk away. However, you can easily trick the system into allowing you to walk off the screen while still on the "string" screen, causing you to go into the black void.