Sometimes it happens that the terminal loses focus: the caret becomes an empty rectangle (instead of the default filled), and my keypresses are not forwarded to the terminal. One of the reasons it can happen is when I accidentally press Alt which focuses the menu bar. In this case, pressing Esc fixes the issue. However, there is another case. I haven't figured out yet what causes it, but neither Esc, Enter, Tab, nor anything else I tried solves it, only clicking in the window with the mouse. The window itself remains focused though, and I can still access the menu using Alt, but even entering the menu then pressing Esc doesn't restore the focus on the terminal.

What causes the issue, and is there any way to fix it using the keyboard?

I've set "Focus follows mouse" (delay 500ms, low) in SystemSettings - WindowBehaviour - Focus. That helps me when I loose focus on current window. Just let the mouse pointer anywhere in your desired terminal window and it re-focusses it after half a second after loosing focus.

As I said, the problem is not that the window loses focus. The window has focus, focusing on other window and back doesn't help, neither is entering and exiting the menu with the keyboard. I have to explicitly click on the Konsole window to be able to use the terminal.

I could finally reproduce the problem. If I press ALT (thus enter the menu bar), then switch focus to another window, then switch back, the focus will not be on the terminal, but the menu bar will not be active either. Neither ESC nor any other key activates the terminal. Pressing ALT again activates the menu, but exiting it does not activate the terminal either. The only way to bring the focus back to the terminal is to click on it with the mouse. This looks like a bug.