You can’t do it this way 
unfortunately.
There is a Merge between the server dataset and the client dataset where this will get lost. You have a few options, replace the grid with a custom BAQView Grid (see other posts about it)
Or you could do a simple Foreign Key lookup and add the field via a customization. Or you could add the UD fields to the LaborDtl table in the Extended UD (as an actual UD field) then populate it as you are doing.