I had an issue with this as well.
I found that the fix was to select ITEM_VARIATION in object_types
So I ended up with
{
"begin_time": "2021-21-06T12:00:00Z",
"query": {
"exact_query": {
"attribute_name": "sku",
"attribute_value": "MYBARCODE"
}
},
"object_types": [
"ITEM_VARIATION"
]
}
Hope this helps!