InputAction applied in a display event handler to a user binding (User1-User20) only returns non-zero immediately if the key is pressed *and* it happens to bound to some other action. If the key is not used in some other binding then InputAction will not return true until the key starts repeating.
It's a small thing and something of an edge case but these are exactly the type of 'unknowable' issues that drive users and addon makers crazy. {F25136}
Bind two keys which are already bound to some other action to the custom action [User10]. Here I will use 'K' (default binding [Compass]) and 'L' (default binding [Headlights]).
Run the attached mission, each time either key is pressed and immediately released two lines will be printed to chat;
<Time> User10 key pressed...
...and flagged by InputAction.
Now unbind 'L' from [HeadLights] after which pressing it will only yield;
<Time> User10 key pressed...
...unless it is held down and begins repeating. The 'K' key which is still also bound to [Compass] behaves as before.
A test for a single keystroke should not return different results just because the key doesn't happen to also be bound to some other action.