I faced the same problem a few months ago.
Then they told me that the fact is that the NPCs died from “starvation”…
Therefore, I periodically run and interact with my guards, putting food in they inventory.
It is also unclear how the slave vat/animal basket currently works - there is no panel with a list of thrals who feed in the radius.
And recently there was a bug when many players’ thrals disappeared for no reason at all. I suffered from this as well.
Funcom are never responsible for their mistakes and do not return/compensate for what was lost!