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Not enough info for a reliable response. Did the URL structure change? Are the previous keywords still getting impressions and clicks? Did they include new internal linking? Are all the (new?) pages indexed? Do you have backlinks from other sites that now point to nowhere? Did they do 301 redirects to new pages?
It’s only been 21 days, so this may be google trying to learn the new site. Or, it could be the designer missed some things. Dive into your GSC and GA and see what is going on.
