Hi Tomás,
With your explanation, it looks to me that the expression does not spatially constraint the operation. A mathematical expression is applied to the FEFLOW's parameter and to all the mesh elements (automatically). So, you need to reformulate the express, eg. to apply the In/Outflow on top/bottom only to specific areas. A tip: Expressions works also with Elemental User Distributions. You can use them for creating the constraints.
Please follow-up with our technical support at mike.de@dhigroup.com for details on the workflows.
Cheers
Carlos Rivera