The form view within the original doc works as expected, but if it is embeded into another doc or is accessed via the published link, the filter() formula doesn’t work (neither in column formulas nor validation formulas) despite:
Referencing the same table
The ‘Include referenced tables’ option being activated
Being logged in with a user who has full access to the original doc
Is this behaviour by design because of some security concerns or is it a bug?
I copied your doc - I can see the difference between the embed and published, but I don’t understand what you are trying to do.
When filtering the related column, you normally put your filter formula in the options, but you try to fill the column with a formula - I don’t think this can work.