Thanks for your reply. It is hard to check directly but I'm pretty sure it is okay, because: 1. this particular component was soldered with hot air and soldering paste (by hand), which makes it hard to not get the pad right; 2. the device does not get very hot in normal operation, certainly significantly less hot than in a similar setup where the paddle was done without vias in itself; and 3., applying a soldering iron to the other side of the vias under the paddle makes the top side of the IC hot enough to burn my finger in few seconds, which means there is very good thermal contact.
If you still think this might be the issue, I can also resolder the component completely, or build a second prototype to check for this.
Best regards,
Sven