Indeed, we were relying on the matlab signal processing toolbox for filtering. I opened a pull-request (add oc_filtfilt by Edouard2laire · Pull Request #578 · brainstorm-tools/brainstorm3 · GitHub) to fix it.
This process is using IIR filter. Otherwise, you can use Brainstorm filter process that is using FIR filter. Just make sure to specify the transition band and check that the filter response correspond to the filter you want.
More info here: https://neuroimage.usc.edu/brainstorm/Tutorials/ArtifactsFilter#Filter_specifications:_Low-pass.2C_high-pass.2C_band-pass
Edouard