This webview behavior seems to be common across a lot of apps. I habitually tap the Safari icon on the bottom right of the webview to get into the Safari app. I had the impression that iOS must require this, perhaps to prevent an app from being able to effectively pollute the user’s global cookies, or to prevent broken UX when the user has some existing global cookie that changes website behavior in a way the developer doesn’t expect. It’s annoying to have to tap out of the webview to avoid repeated logins, but I could imagine it being the least-bad solution to a more complicated problem. Maybe a less confusing way to present it would be for the webview to open in a private tab?
I'm skeptical that Apple is imposing this on them. Many apps (Reeder, Reddit, Discord, Toot!) have an option to toggle between opening links in an in-app webview or Safari.
Reeder's options are "In-app browser," "Safari," and "Firefox" on my device.
