You are using an Add Form screen. The Add Form is never connected to any particular row. What column are you expecting to use if that screen is not connected to any data?
But, an Edit screen is connected to a row because you are editing an existing row. An Add screen is not connect to any row, in any table, in any possible way. You can’t pull something from nothing. Which specific row of data would you expect that column value to be coming from? Since your Add Form is not connected to any row, there is no column value for the label to use.
Also, since you are attempting to use a value from a computed column, that computed column wouldn’t have a value yet, since the row has not yet been added to the table.
There’s probably other ways to achieve what you want, such a a normal Form, which has access to an underlying parent row, or even a custom form, but the Add Form is fundamentally different because it is not attached to any data (with the exception of user profile data)
Yeah, I think if the top level screen was changed from a List Layout to a Details layout, then if a Form button - or a button that opens a form screen - was added, then they’d be able to access the column values in whichever row the parent details screen was attached to (including any computed column values).