Well, it's pretty much deceptive advertising on Samsung's part. I forgot the exact article I read but the SamMobile article below pretty much says the same thing...
You have a 45W charger and the phone will handle 45W charging. But the only time you see anywhere near 45W charging is in the front half where the battery is really low as you said. And then basically Samsung is throttling the charging. IMO most likely to prevent high temps.
So they are throttling CPU performance to avoid heat. And they throttle charging even though they advertise 45W charging to avoid heat.
Otherwise, if you have a 45W charger and your phone is capable of 45W charging, then you should be able to take the full benefit of 45W charging from 0 to 100% and not pay the extra money for the 45W charging just to charge your phone 3 minutes faster than if you used your ole 25W charger.
While you see competitors phones with crazy 120W charger in video going from low to full in minutes.
https://www.sammobile.com/news/samsung-unimpressive-45w-charging-is-both-good-and-bad/