The Bestiary rounded up: with 210 of 211 species it said «100 %», and the achievement, which needs all 211, did not unlock. The pause menu now shows how many you have out of how many (for example, «210 / 211») and the percentage never rounds up. As soon as you record the missing one, the achievement unlocks on its own.
Thanks to sk8heaven. Steam downloads it automatically. Thanks for playing!