He was not reduced to acting ensign. He was an ensign. In "Unimatrix Zero", he was promoted. Like Tuvok was, and like Harry should have been.
Tom Paris:
Prior to 2368: Graduates Starfleet Academy, commissioned as an
ensign on the USS Exeter, promoted to
lieutenant junior grade.
2368: Discharged from Starfleet after his involvement in the death of several cadets comes to light.
2371: Reinstated at his
previous rank by Captain Janeway.
2375:
Reduced in rank to ensign by Captain Janeway.
2376:
Reinstated to the rank of lieutenant junior grade by Captain Janeway.
Tuvok:
Prior to 2349: Reinstated in Starfleet at the rank of
Ensign, serves on various starships.
2349: Transferred to Starfleet Command.
Between 2349-2371: Promoted to
Lieutenant.
2374: (Field?) Promoted to Lieutenant Commander.
Tom Paris was granted the rank of Lieutenant JG by Starfleet Command, Janeway reinstated this and never promoted him
beyond this rank, although she did briefly remove it.
Tuvok was granted the rank of Lieutenant by Starfleet Command, Janeway chose to promote him
beyond this, possibly as a field promotion (ala Wesley) that would not follow him off
Voyager unless ratified by Starfleet Command.
Kim was granted the rank of Ensign by Starfleet Command, Janeway chose
not to promote him beyond this for reasons that are unclear in universe.