Ruby, 78 bytes

f=->n,r=[[]]{r.find{|q|q.sum==n}||f[n,r.product([*-n..-1]).map{|x,y|x+[y*y]}]}

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Old version, with squares in ascending order (wrong)

Ruby, 76 bytes

f=->n,r=[[]]{r.find{|q|q.sum==n}||f[n,r.product([*1..n]).map{|x,y|x+[y*y]}]}

Try it online!