I occasionally have this issue with world comp - it usually just means you have actors or foliage that are part of that tile but far away enough to skew the original bounds.
To avoid this, be sure to work within the right tile while working with world comp (in world comp, right click the tile you want to set dress and select make current to have things load and unload with it).
If you aren’t careful, stuff that you think is fine in-game will unload in unexpected ways (say you have a cactus meant for a desert but it’s technically placed in a distant forest tile by accident. The stray cactus will disappear when that distant forest chunk unloads since it was in the wrong chunk to begin with).
If you need to move over actors that are improperly placed, you can select them and right-click the destination tile - there should be an option for “move selected actors to level”. This moves them to the right tile.
I hope this makes sense!