I’ve found that when adding an input mapping context, such as returning to gameplay after a cinematic, UInputTriggerPressed will trigger if the input action is currently being held down. On the first tick after being added the trigger’s LastValue is always unactuated, which will cause it to trigger if the mapped button is depressed.
One way I’ve found to solve this problem is to force the trigger to always return ETriggerState::None the first time UInputTriggerPressed::UpdateState_Implementation is called.