Fabio, I'm not sure why you're calling this a bug when it shouldn't have worked in the first place.

PROMPT FIELD FieldName

Specifies the field name from an open table whose records become the items on the menu. The menu 
contains an item for each record in the table. When the menu is activated, the tables' work area is selected. 

Tip   You can take advantage of Rushmore Query Optimization if you set a filter on the field specified with 
PROMPT FIELD used in the menu. For more information on Rushmore Query Optimization, see SET OPTIMIZE 
and Using Rushmore Query Optimization to Speed Data Access in Optimizing Applications.

FieldName can also contain multiple field names and expressions concatenated with the addition 
operator (+). FieldName can also be the name of the field in a table open in another work area or a 
user-defined function. 

There is no limit to the number of entries that can appear in a menu created with PROMPT FIELD.

So, first, it's supposed to be a field name, not a function call.
Second, it's documented to change work areas, but the work area you're pulling from is in a different datasession.

Frankly, I'm impressed that you got it to work.