I have been able to track this bug/property back to MechJeb. MechJeb has a functionality to lock all keyboard inputs when hovering over a MechJeb window. So if you happen to hover mouse over a MechJeb window while pressing or even after you just pressed the EVA button. Then your keyboard inputs are locked. They can be unlocked in the input locks section of debug menu. To avoid input locks: move Mechjeb windows away from the EVA button, hold your mouse cursor still for couple seconds after pressing the EVA button to avoid hovering over a mechjeb window. If the lock still happens, unlock via debug menu.