Raihan Lutfianto

From what I understand, it is possible numerically. It is still considered close to zero. However, I recommend you to recheck:
- Inlet boundary condition
- Outlet boundary condition
- Solution methods setting
- Residual setting
- Mass balance report definition
- Any reversed flow inside your model

In my case, the solution is also negative. Again, if the overall physics does makes sense, then you do not have to worry about it.