When dropping items or harvesting crops, they will fall under the gardening plot directly onto the terrain in certain spots, particularly around the edge of the plot and in the spaces between plant slots.
Please see the video for a few examples:
https://youtu.be/D5BQNY1E6eE
-Drop an item at different spots on a gardening plot and you will notice that in certain spots it drops straight through the plot onto the terrain below.
OR
-Harvest a crop of small vegetables such as tomatoes when standing on the area between plants, or on the edge of the plot and you will notice they will often be placed under the plot on the terrain instead of on top of the plot as they should. (see video above)