What is "Solder Soaking"?
It has been mentioned in the comment by Manu But I'll elaborate here in an answer:

When placing thermal vias underneath a component with a thermal tab and these vias are open, then the solder will suck through the vias because of capillarity. When this happens, it can happen that's there's not enough solder left in order to make a reliable bond. The result is that the thermal enhancement effort makes things worse.

The solution is to close the vias through plugging, e.g. with silk screen (=tenting, not recommended as there are no guarantees) or dedicated plugging.