It's definitely a problematic pattern - you can continue adding non-grid prop items to a group to your heart's content and multiple groups together as long as they also have a maximum of one gridded group, but as soon as you try and add two grid prop groups you have to switch to blueprints - which is fine as long as you know that's how it works - not in game at the moment so can't check, but a pop-up message to say save as blueprint instead would be handy if it's not there already - otherwise it just appears to be broken.
Groups being separate types of things from Blueprints is good, one's more permanent and transferable than the other. But if you want to, say, have the front of a pitched roof overhang a wall the only way to do it is to put the roof on the building so it all lines up together, split the roof off in-situ, do your edits and then save the roof and the walls as a blueprint - which is probably what you want in the end anyway, it's just a bit counter-intuitive in terms of steps to achieve the end result.