EDIT.
Scratch that about not checking, cause some circumstances changed like they should've changed in first place and now it is working fine. Sry about inconvenience on my end.
After some tests, the cinnabar dust+empty cells process is always prioritized if you don't have any cells and cinnabar dusts inside it before you start the cinnabar process..
If you have empty cells inside it and put cinnabar dust later, the cinnabar dust will be ignored and all empty cells will be converted to hydrogen cells.
Or if you have an on-going hydrogen process and put a cinnabar/empty cells into the electrolyzer before it finishes, the cinnabar will be ignored as well..