Codex No.17 is not unlocking. I believe it's related to the spider-like enemies located at the far left and right of the starting area in Stage 3. I’ve tried defeating them using different methods such as changing suits, using punches, and charm abilities, but the codex entry still remains blank even after returning (Codex No.17 is still not filled in).
Notably, after defeating the enemies and returning to the same spot, they respawn, unlike other enemies which disappear once defeated.
This behavior was confirmed on Patch 1.02.