Slice frames allow users to identify in 3D views which slice view an image corresponds to. In the future, the frame will be interactive, so that you will be able to click and drag it to translate or rotate the slice. It is a feature, not a bug, because it can be turned off, as described here.
If you cannot hide a slice frame then please provide detailed list of steps to reproduce it and we’ll investigate.
@lassoan I haven’t really investigated much but I recall also seeing these frames hang around, at various times, typically after closing a scene in some unusual situation.
Here for example with today’s preview build I got this after dowloading MRHead, linking the slice views, making them visible, switching to dual-3d, linking the 3d views, and closing the scene. There may be simpler ways to reproduce but this is an example of what I’ve seen before. If we don’t already have one we should file an issue for this.