After some local testing, it looks like the main HTML reports can only handle one brainsprite figure at a time. If I request the ALFF figure, then the ReHo one, only the first run's ALFF figure will show up. If I request the ReHo one first, then only the first run's ReHo figure will render.
This is probably a bug on niworkflows' side, since the Report class is ultimately imported from there. However, it could be something that was resolved in a more recent niworkflows version, or they might even be unaware of the problem.
Summary
After some local testing, it looks like the main HTML reports can only handle one brainsprite figure at a time. If I request the ALFF figure, then the ReHo one, only the first run's ALFF figure will show up. If I request the ReHo one first, then only the first run's ReHo figure will render.
This is probably a bug on niworkflows' side, since the Report class is ultimately imported from there. However, it could be something that was resolved in a more recent niworkflows version, or they might even be unaware of the problem.