So everyone probably knows that Apple throttle their CPUs to save power and battery. They have done this on their Macs for as long as I can remember. They drive the temperatures right to the TDP limit to keep the fan noise to a minimum. That is why you can game the first half hour on an iMac but after the silicon warms up it starts to throttle. This has been proven in several benchmarks, even I saw it a few times when I owned one of their iMacs.
Apple also perform this practice on their iOS devices, as batteries age the capacity and amount of charge the battery is capable of holding gets lower. To prevent unexpected shutdowns during work-load spikes Apple may throttle down CPU performance to save power.
In a technical way this is just fine, but the problem here is that Apple doesn’t tell you about this. At least not in an easy customer accessible way. Maybe they have written something deep-down in some release notes somewhere but that doesn’t really count.
When the news from reddit broke Apple was quick to offer cheap battery replacements, as a new battery solves the problem. This also indicates that they knew this was as issue but gambled on that customers wouldn’t find out, or care about it.