For instance, the first version of Tweetbot for iOS 13 wouldn't allow you to choose a dark theme if iOS was set to use a light theme.
This was quite frustrating because I like Safari in light mode, but I like Tweetbot, iBooks, Kindle, etc in dark mode. This always has always worked and I'd see a different behaviour as a regression. That said Tweetbot fixed the issue very quickly (in a day).