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.
One more small related issue which is surely unintentional:
Moving around multiple items at once and dropping them in occupied spaces results in the "overlap" dialogue persisting beyond when the items return to their initial positions. The dialogues pile up if this is done more then once.
When moving only one item at a time, the dialogue dissappears after the items are dropped.
