Hi!
When you play backwards a tween, it will play backwards from its current position, loops included, until it reaches the start of the tween. In short, the tween is treated as if it was a movie timeline, where the loops were part of the timeline itself. This means that, even if you set infinite loops, if you play backwards after 3 loops only 3 loops will elapse before the tween reaches its beginning and stops.
You could use a trick here though. After creating the tween with infinite loops, use Goto (or DOGoto if you're using a target shortcut instead of a tween reference) to send the tween to a time position that is, for example, after 10000 loops. That way, when you play backwards, it will have to go through 10000 loops before it stops.
Cheers,
Daniele