I have had the exact same issue as you. Grouping symbols, turns them invisible, even though the object can be snapped onto and the cube appears around it. Classes and layers are visible, but this still happens. I believe this to be an issue with the software v2017 itself, and has not been resolved as of yet. What I have found that works to make them all visible again, is to ungroup the grouped symbols, and then undo your action, using the undo command. This hasnt failed me yet! Another thing i have realised (has been mentioned before) is to enter the group, and enter the symbol edit window, and then exit. This makes the symbol visible again, but only that symbol. other symbols will still be invisible.
PS. lowering the resolution doesn't work. My workplace doesn't have the most powerful computers, so I have to always work in the lowest res, and i've had this issue since upgrading to v2017.