Damn. Let me ask you this: do you have the always-on display turned on on the 14 Pro?It worked on my old 13 Pro and my 14 Pro.
I'm asking because I sleep with my 14 Pro on my nightstand and when I wake up the display is completely off (I have always-on turned on, but it eventually turns off if you haven't touched the phone in several hours) and light mode is active.
However, I'm wondering if dark mode doesn't trigger automatically because the always-on display never goes totally dark since I'm usually using my phone a ton around sunset. Maybe today I'll try keeping the phone in my pocket or face down for a few minutes before and after sunset. Thinking that maybe if the always on-display isn't active, dark mode will trigger properly.