woocommerce custom product type not updating stock after purchase

For a customer, i have created a woocommerce custom product type "event".
For reasons of simplicity, i gave this product type a single custom tab: "event settings".

In this tab, the user can insert all relevant data, such as event date(s), start and endtime and so on. on this tab i use regular woocommerce fields as well, such as price, stock and so on.

Everything works just fine, except the _manage_stock part. When i make a purchase, the stock is not updated. I have made a checkbox to mimmick the _manage_stock checkbox and save it to the database with value yes.

When i inspect/var_dump the database, the correct values are there: the value for _manage_stock = yes and the value _stock is 8 (in this case).

Nevertheless, the stock is not updated and when i switch to a simple product, the _manage_checkbox is deselected, even when the value in the database is "yes".

I have no clue why it is not working.

Anybody?