We are aware of this bug but have not yet been able to fix it. It's a problem with chaseable imps in general, not anything specific to a particular quest.
- If this is a timed quest, don't worry about it—you'll miss out today on a minor reward and may continue to miss out for a few days, but eventually this quest should go away with no permanent detriment to your game.
- If this is an untimed quest, please try checking in on the imp every day. Imps can (but might not) change positions on each new game day, and likewise for the terrain, so hopefully one of these days the imp will become unstuck and you can successfully chase it and get the item(s) it's carrying. Some players have reported it taking up to a week, some only a day or two, and some I'm not sure have ever gotten unstuck, but hopefully it will be on the shorter side for you and you won't have to wait for a fix.