Help! How do I roll forward the latest value

How very strange. I cannot replicate this behaviour, when I use your code it still allows me to create proper one-to-many relationships in the right direction.

The only thing I can suggest is to modify your code a little. You don't need to add the [Calendar] column as the CALENDAR function already gives you a [Date] column. Also, there is no need for the SUMMARIZE - the table is already at the granularity of one day, which is what you need, so there is nothing for SUMMARIZE to do.