I had the V16 DLC and now it’s gone from my library and my purchase history and I’ve made V16 engines before but they default to a V10
That’s highly unusual; does the DLC still show up in your purchase history?
This isn’t something we have control over; something like this is a Steam issue. If the basic fixes (verifying Automation’s game integrity, clearing your Steam download cache) don’t work, you will have to open a support ticket on Steam at help.steampowered.com.