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Usually things like redesign, relaunch, and content get mixed a lot.
I don’t think SEO companies should do website redesign as that’s for developers.
Redesign usually should be a design change not content, and in some cases the whole CMS.
Usually the rankings drop after a redesign is normal but should pick up after a while if the content was not changed dramatically. If categories, urls or structure was changed a lot if might be the cause of the ranking drops as well, as that might affected how the link juice is split internally.
So without the details no one can really tell, but hopefully you recover soon.
