How to make block disappear with some chance when its destroyed?

Is it a custom block? Like a custom dirt block for a dimension you added, or biome?

Also, be patient. People would most likely respond later on.

execute command as entity:(/kill @e[type=item,name="<CUSTOM ITEM NAME>",r=1])

create a json loot table file with trigger chance

something like this

"conditions": [
{
"condition": "random_chance",
"chance": 0.1
}
]