It happened to me several times that I entered my password into the last active window as I started to type it, although kscreenlocker was still in its "grace period" during which it doesn't require a password yet. As long as the grace period (as specified in the KCM option "Require password after locking") hasn't passed, the password field should be hidden to reduce the risk of accidentally entering the password into an application.
That should already be fixed: https://quickgit.kde.org/?p=plasma-workspace.git&a=commitdiff&h=70d3ec9f724bdbe40a84ce78b9ed9e41811db6a4