The double furnaces do not use the Steel Plate Productivity Technology.
This block in data-updates.lua fixes it.

if mods["space-age"] then
table.insert(data.raw.technology["steel-plate-productivity"].effects, { type = "change-recipe-productivity", change = 0.1, recipe = "double-iron" })
end

Thanks

Can you submit a PR? :)

Raid, i've got this fixed on my end but i'll hold off on the PR till you reply to the post about reducing the cost of the electric research to be back in line.
As i'll do all as one PR and version change to make it easier.