NULL value in DB clients.currency_id

As the title states - we have several clients that have properly selected currency via web UI in Clients - Additional Info - Settings - Currency and yet the vast majority of these clients (we are talking 85%+ of all clients) have a NULL value in the database in clients.currency_id .

Is this a known thing? Are we doing something wrong?

The thing I am trying to achieve is to export certain data out of the DB in order to make a proper report but since these fields are NULL I cant get everything out. I also planned to hard code into the report so that it would display the standard international currency code ( based on currencies.code in the DB.

Clients with a null currency use the company currency by default.

We advise accessing data through the API rather than reading directly from the database.