iPad and iPhone owners are reporting that the keystrokes on their stock iOS keyboards are getting stuck after updating to iOS 8.3. The problem occurs when a user is typing on their stock iOS keyboard and a letter pop-up freezes and obstructs their display.
It is unknown why this is happening, but users on the Apple Support forums seem to think it stems from having the Grayscale filter enabled to dim your display. You can disable this feature by navigating to General > Accessibility > Zoom and tapping the "Zoom" slider so it is OFF / White. If this does not work you can also try rebooting your iPad.