I recall a community discussion in recent weeks of some nuances of this functionality, but I don’t find it now.
Meanwhile, you can likely incorporate the [CONTEXT](AppSheet function list - AppSheet Help function into the initial value expressions to govern what result they return when a row is created via the form view you mention.
Initial values will be applied to and saved for all columns of a row added to the slice regardless of the rows included in the slice.
When updating a row in a slice, all column values are updated regardless of the rows included in the slice, but only the columns of the slice will have their updated values saved. So the various expressions in your row can use values of all columns of the full table row, but only changes to the slice columns will actually be saved.
Thank you, @Steve , for clarifying the issue and to @dbaum for the doc enhancement suggestion.
I have added it to Slice columns, as this topic is cross-referenced from Slices: The Essentials. If you come across another section where this specific knowledge would be important and relevant, let me know!