I’ve had my phone since release day and optimised charging has been working fine for weeks. Just not worked for the last two days. I’ve used optimised charging on my previous iPhones too ever since apple introduced it and actually never had it fail before. So in general I would say it’s incredibly reliable. just this time something has gone wrong
I wonder if an IOS update did something to a specific App where it now works more in the background & has ‘told’ IOS to learn your schedule again?
Have you noticed your battery being used more than when you first got the phone?
Either way, I wouldn’t give up on it. Optimisation is a smart feature that should help your battery in the long run, especially if you‘re not using ultra fast chargers.
Wait another couple of weeks & see if it starts again. & don’t toggle it on/off as this may reset it to start its 2 week ‘learning’ mode again.