Dropping any amount of items of something on the ground and picking up any amount of items that are less then what is currently on the ground will not change the texture appearance of the items, by having a full inventory so you can only pick up fewer items.

I discovered this when I was harvesting some bamboo, got a full inventory, dropped a full stack of bamboo on the ground to pick up one single bamboo, realized my mistake and waited to pick up the bamboo to get a full stack again. Then I saw that the one bamboo on the ground was still looking like a stack of items.

I tried to recreate this and it works every time.

It can be easily recreated by filling your inventory with items (I filled every slot with one cobblestone), then get a stack of any item +1. Drop the stack of items and replace the empty slot with another item, pick up the items and the item on the ground will appear as a stack.

I was able to do this with all the different item on ground appearances (full stack, half stack, etc.)

I also tried this in 1.14.4 without OptiFine, I got the same result, I was still able to recreate it flawlessly.

(The file was heavily compressed to fit the 10MB limit)