Wrong price list in the Sales Order Line

Hello, experts,

I can't figure out the problem, so I'm just gonna take my chances here.

I can't really reproduce the problem consciously, so it's very difficult for me to find the error.

In my main price list, which is stored in the customer master (and is also correctly adopted via the defaulting rules), I only have one single position, which sets a certain article type to a price of 0.00, then 2 secondary price lists are claimed via the hierarchy.

Now I have had the problem for a few days that assigning the price of 0 via the article type in the main price list no longer works. This happens again and again in many orders, but I cannot find any similarity between the orders.