Springs are just a piece of tempered/heat treated metal. It's not unusual to have a spring that isn't properly heat treated or had a slightly different composition of the metal so it didn't take the heat treatment the same as others. When this happens it just "sags" as you've described.
Spring shops all over the country often deal with these by heating in their ovens, "re-arching" and heat treating again for "temper".
I'd give the replacements a chance before automatically condemning them for their looks
In closing, it could also be possible that the shop that produced the springs mis-marked a few and the ones installed on your trailer, while bearing the proper part number, were for a lighter axle. Mistakes do happen even in the best of companies.
Hope the new ones work well for you.