Like a lot of other people who I've seen posting on this issue, I like decorating the buildings in my settlements rather than just have empty shelves and so on. The fact the ability to decorate is there and that we can build stuff like shelves shows that decorating is suppose to be a part of this game. However a lot of people are reporting that the items they place on surfaces like shelves and units and randomly falling off or clipping through objects when they leave and come back. This has never happened to me and I couldn't work out why I was getting so lucky with this until I discovered something by accident.
For the first 100 or so hours that I played this game, I somehow overlooked the fact that you can place items on shelves and so on in workshop mode! I have no idea how I missed that but I did and only discovered it when I accidentally clicked on an item in workshop mode and I was allowed to move and place the item with ease.
The way I had been using to decorate my houses etc was to use the "grab" mode like you can do with dead bodies. Admittedly this is a trickier method of decorating and you have to fiddle a bit to get items turned round or sit upright etc. However using this method has proved that my items NEVER move from where I place them. I have only encountered a small number of items I couldn't successfully place like the globes as they just won't sit upright no matter what I do! Things like glasses and bottles can be tricky but entirely possible. It just takes a bit more patience and thought along with a dose of care when moving things around so you don't knock other items over. It's not like workshop where items can move "through" one another.
I have all my house extensively decorated and never have problems with items staying put. It IS a little more time consuming but it's a sacrifice I don't mind making for the ability to make it work. I even tried the workshop mode to place a few items and when I left the settlement and came back it was ONLY the items placed in workshop mode that fell off the surfaces.
I'm sure I'm not the first to discover this but I'm not aware of how many know about it and having seen other people disheartened with the decorating aspect of the game I just thought I'd share this in case it is of use to others.
I'd share some screenshots if I knew how to get them from my Xbox One onto the laptop to post here.