Many Christians of different denominations have reported encounters with demon-possessed individuals. Catholic exorcists are an obvious example. In the mission field, reports of demonic possessions in the 10/40 Window are not unheard of. An interesting first-hand account by a SDA missionary can be found here. Even Baptist pastor John Piper has had his fair share of experiences with the exorcism of demon-possessed people.
Have Jehovah's Witnesses ever reported similar encounters with demon-possessed individuals in the mission field or in other contexts?
Jehovah's Witnesses preach in over 240 lands, collectively reporting over 2 billion hours in the ministry annually. I'm sure there are many such encounters with demon-possessed individuals. However, these stories will be quite rare because (1) demon possession is generally rare, and (2) Jehovah's Witnesses don't aim to spread fear of the demons by telling such stories. The ministry is about telling the good news.
I was able to find a couple of experiences published in old Yearbooks and Watchtowers.