I was having the same issue and I was able to solve it in C++ by overriding NativeOnInitialized and setting bAutomaticallyRegisterInputOnConstruction to true. It seems like it’s supposed be set to true during compilation if the blueprint requires input handling but something must have broken in that flow.
void USomeChildOfUUserWidget::NativeOnInitialized()
{
// HACK: This is supposed to be set at compilation time but it is not working as of UE 5.5
bAutomaticallyRegisterInputOnConstruction = true;
Super::NativeOnInitialized();
}