I have had spirea for a number of years, 2 Goldflame and one dark green. The leaves on the Goldflame are changing to dark green but I haven't fertilized. Maybe it caught a bit of seaweed solution, but I don't think I went near these with that.
Thank you for your answer.
According to Clemson University's Agricultural extension, it's normal for the leaves on the Goldflame cultivar of Spirea to change to a green hue in the summer. It's surprising that you haven't had this happen in years past -- but it could be simply because the plant is more mature and well-established. It should change again in the fall, too, to orange-copper or orange-red.
These articles contain some information about Spirea: