Bah, nevermind.

Delay Validation set to True is working for the remainder of the packages I am creating.

However...

What made me think DV was not involved was that my first package had DV set to False. As I was comparing my first package (that worked fine and had both an expression to set initialcatalog and a hardcoded initialcatalog in the connection string) to my second package (initcatalog would get cleared when I added an expression to set initcatalog) both had DV set to false.

So, somehow, my first package was able to have an expression for initialcatalog as well as the connection string; not sure how that happened.