I encountered similar behaviour - when I group some layers the group disappears from the viewport. I did find it though way outside of the art board. When I moved it back to place and ungrouped, the released layers jumped away again, this time in the opposite direction. Seems to me, that the layers keep some kind of coordinates, that become relative to the group container when you group them. Also I realised, that the groups with this behaviour are automatically named "Constraints group" in the layer panel, not just "Group" as usual. I didnt find anything about Constraints groups in the help topics. Is this a standard feature?