If you are using an IBus input method to enter text into an application in GNOME (normal for Chinese, Japanese and Korean users), and then the screen is locked - whether you lock it yourself, suspend the system, or it is locked due to idle time - then after the screen is unlocked, input with the input method will no longer work. The input method will also not be shown on the right-hand side of the panel. If you switch focus to a different application and then back to the one where you were inputting text, it will work again.
We suspect this happens because input methods are disabled when entering your password to unlock the session, and somehow the state isn’t being properly restored after the password is entered (until a focus switch clears it up again).
Bugzilla report: #2062619
As mentioned above, just switching focus to another application and back clears up this problem.