The problem is the same in all of my applications. To reproduce it, in iOS 26, set the dark mode in the Brightness and display settings and in Accessibility, Display and text size, activate Increase contrast and bold text. With these settings, all the controls will be surrounded by a thin white line. When in the app a keyboard is dispayed, the thin white line does not appears correctly around the keyboard like in the capture joined, it is present on top and partially on bottom but not on sides
Update: to reproduce it in iOS 26.1, the options to set are in the settings "Display and text size" enable "display borders" but disable "increase contrast",
so as I like the new border option, I enable the "increase contrast" to avoid the bad display