As a Roblox developer, it is currently impossible to have large parts that actually stay where you place them when you test your game online. If you place a large part, anchored or not, in studio you’ll notice that it stays in the same position if you save the place and open it again in studio, but it shifts out of place (more so the larger it is) in online mode.
I may be wrong, but I’ve seen it mentioned somewhere that this is because part positions are sent to clients with less precision than they’re stored in the place’s rbxl in order to reduce bandwidth usage. Makes sense. I feel that anchored parts, however, should have their positions sent to clients with full precision, as they’re not intended to move. In the rare event that an anchored part is moved, it’s likely to be done by using a tween, in which case the end position could be sent to the client and the client can interpolate it (as I think it already the case).
Again, there’s a fair chance I may have misunderstood what was said or be entirely wrong on all accounts here, but on the off-chance I’m not, this fix would be greatly appreciated by all of us who use large parts.