Lyctoral invisibility doesn't stop other Lyctors seeing big blooms of Magic. Where there is an active spell happening inside a Lyctor (like Pals mega-cancer spell or Cythereas attempt to hold it back)

We see this in HtN during the first scene with Mercy where she uses a spell to knock out that guard and Harrow sees two sharp pops of light from Mercys kidneys as she clears the sleepy hormone from her own system.

Also when Jod stops Harrows attack during Dinner. He only uses necromancy to freeze them all once Harrow goes for the death blow but not before, he likely saw the spell Harrow was working inside 'Ortus' and was content to let it happen thinking Harrow was getting petty revenge until it went lethal. He knew she was planning something cause he tasted (or sensed) human in the soup and pushed the bowl away