I updated to 1903 last weekend using the release preview ring, and today after a cumulative update, .NET was gone.
I restored Windows 1809 using the previous version of Windows 10 option, mentioned above. However, .NET is still broken. Never encountered anything that would corrupt even the previous installation as well. Amazing bug.
I stopped using Fast and Slow ring builds a while ago, but I guess I am joining the people who will never touch an insider build ever again. Sad day.