When you **deploy** a plastic explosive it will always sink half into the ground and it won't show up in your inventory afterwards. After arming it (if the game lets you, but that's another story...) it still doesn't show up in your inventory.
If you just put the plastic explosive on the ground without deploying it and then arm it, it won't sink into the ground. Using this method it will show up in your inventory as well.
I've also made a video about it:
**[[ https://youtu.be/A9QnUGZwa5M | >> VIDEO << ]]**
{F2992320}
This bug has been around for quite a while (if not from the beginning).