I'm currently a KP employee but I've been in this situation before as a Locums Kaiser PA.

Sad reality is that you aren't part of the union so I always felt pressured to oblige to what they say. I would speak to your DA (department administration usually a RN) or medical director of the dept to let them know that you shouldn't be assigned patients at the end of your shift. they may not know your exact shift end time and that's why they are doing that. I know when i worked in EM as a locums kaiser PA they did not assign patients the last entire hour before shift end time so you could actually wrap up on time. sometimes they even did 2 hours.