Running Ver. 10.7.19
A observation as this option is not working how I assumed it would.

I was running a route for Galapagos to Marquesas for a friend.  Route is approx. 18 days.  I was curious how extend wind in time worked so for testing I downloaded a GFS Grib from Saildocs for 10 days.

With "Extend Wind in Time" checked I ran a route and it took me all of the way there.
When looking at the optimal route I expected the Twd and Tws to freeze after the 10 day grib "ran out" and show as a constant number equal to the last entry in the grib.  It does not display as such when viewing the optimal route which is what is prompting this post.  How is Extend Wind supposed to work?

This is probably intentional but it took me a minute or 10 to figure out where my little boat icon went.
When animating the weather either, manually stepping through, or using the Play option, when the grib runs out it loops back to the start.  When viewing the progress of the boat along the optimal route the boat also loops back to the start, so there is no way to view boat position along the optimal route past the last time stamp in the dowloaded grib.  I suppose if the wind is faked (guessed) where you boat might be at that moment may not be very relevant.