Thanks for your answer.
It seems like my inventory export is not working.
I attached a screenshot where you can see that the values in the inventory are not correct. There are two tables. The left one is the inventory export and the right one is a product export. I matched the product IDs per line.
Even though it is the same product id and the main inventory (is_main = 1) there is not the same value in the "_stock" and "stock_quantity" fields.
The stock from the product export is correct. The stock from the inventory export is always zero.