by default, if tsk parameter is empty, the launch will not be associated to a Task (where variables are defined).
Resolution
As explained in the documentation, if the "uxadd fla" command has the TSK argument transmitted, the launch will be associated to that Task and will use the Task variables instead of Uproc variables.
TSK=
(Optional) Enter a task code or * (wildcard accepted except for uxadd). Incompatible with UPR, SES and USER.
VARTSK | NOVARTSK
(Optional) Variables:
VARTSK (or Y on OS/400): (default) The launch is created using the values of variables of the corresponding task (if it exists).
NOVARTSK (or N on OS/400): The launch is created using the values of the variables of the Uproc defining the launch.
Additional Information
An example of the minimum necessary syntax to launch the task called IU_UMNT via "uxadd fla":