Hi 2manybeds,
I know from talking with them that the Saatva manufacturer takes care to design their mattresses in a way that is less likely to develop body impressions that are that deep so it would be somewhat unusual for this to occur … especially that quickly. You can see an analysis of their design in this post.
I would also make sure that the sagging isn’t connected to the support system under the mattress (you could put it on the floor to test this).
Their warranty considers 1.5" or greater body impressions to be a defect and if you have measured the impressions correctly and this is happening within 3 months then I would talk to them about returning it because you may have a defective mattress.
Phoenix