In general, fungicide applications made late in the window (at around 50% flower) are not as effective as applications at 20% flower because early infection tends to cause the most yield loss.
Also, fungicide does not provide a curative benefit, so any infection present before application will not be stopped. A late application may stop subsequent infection, but infection that is already present can spread throughout the plant and from plant to plant in cases of extreme lodging, reducing the effectiveness of this late application.
This year, growers may see good reason for two applications (7 to 14 days apart as specified on the product label) if conditions are good for fungal growth and the crop flowers for a long period.
This article includes more info and a table of products with split app options: Does late-window spraying for sclerotinia stem rot pay?