The Flatline is a Dead Short Only Tray, and never stopps the discharge. Just resistors on the cells.
The Smart Tray's Dead Short Mode will pull them down to nearly 0 then kick off for a moment and kick back on. So it doesn't "Hold" them like the flatline does.
Here's a link to the Frying battery thing. EVEN if the Smart Tray has something wrong with it, it won't HEAT UP BATTERY packs. You HAVE to put them in wrong, NOT JUST THE POLARITY, thats only half of it, but the actual posisition of the place the ESC connect have to be correct. Here's a link with a picture.
Also a Picture inside most Boxes.
I get this ALL THE TIME, I've check DOZENS of Smart Trays that report EXACTLY the same problem, and the fact of the matter is, there is NO way for the smart tray to heat up a pack on its own.