Coda Form Validation Issue

I’ve been playing with form validation to build a calendar/scheduling functionality.

I’ve enabled hidden columns and lookups, but the form validation only works within the coda interface. When I preview, copy link, or embed, the validation stops working properly.

For reference, this is the validation formula I’m using:
[All Dates].filter(Date=CurrentValue).Status!=“Booked”

[All dates] is a table (on a separate page from the form if that makes a difference) that codifies all dates that have been “booked.”