May I ask where did you purchased this used iPhone? I know that refurbished iPhones from Apple come with a new battery at 100%, and I would expect that to be the case at most refurbished retailers.
Your iPhone battery health is expected to drop about 1% for every 50 charge cycles, and the decline isn’t always linear. Meaning some months you will lose more than expected, other months none at all.
Also when your iPhone was at 231 charge cycles, it should have been at 95% health, so you were ahead of your battery at that time. It’s possible that now your iPhone’s battery health is declining at a normal rate.
Keep in mind, all rechargeable lithium batteries chemically degrade over time. When your battery health reaches 80%, have the battery replaced.