This is vanilla Minecraft in version 1.16.4, I've tried this in the nether as well as well as with other items, and this does not change. Additionally, if I throw down stackable items with non stackable items, a lot of stackable items before they stack, or full stacks of stackable items, then the same. I have not tested this in other versions, but it did also happen in Optifine 1.16.4 HD U G5. This does not happen in survival.

Video: https://www.youtube.com/watch?v=vgWok1HCAwg