Do you mind clarifying what the issue is? Is it that you think only one should be movable at a time, or that they are all moving, and something else is happening that shouldn't?
Working with the garden on a mobile phone, you have to zoom in to place items precisely. Otherwise, your finger covers 3-4 items already there. However, upon zooming in, it's very likely that you'll just end up dragging around 2 items that have been placed. It doesn't really feel like an intentional feature, especially since I can't replicate it on PC.
