i wrote that too and is marked wrong.. have reported. 31 Oct 2018

In meaning it's the same but grammatically that would look different. 바지 없는 곰은 뛰다

That would be "바지 없이 곰은." The position of "바지 없이" determines what it modifies. In this case, it's an adverbial phrase modifying "is running."

It's not incorrect but even though "The bear without pants is ." and "The bear is running without pants." have the same meaning, they are formed differently. It's the same in Korean. The sentence you wrote would be structured differently.

"The bear without pants is running" would be 바지 없는 곰은 뛰고 있어요, so 곰은 바지 없이 뛰고 있어요 is like saying that the bear runs even without pants, or that regardless not having pants the bear runs, the difference is 없는 (adj/nominalized descriptive verb) and 없이 (adv)

so jumping should be accepted