Hi.
I am on a Mac with, I think, the same settings as the ones you describe (light mode in Apple > System Preferences > General > Appearance, and light mode in Obsidian), and I am not seeing the same as you if I enable the Pisum theme. I am running the beta of macOS 12 Monterey, so that might be significant.
Don’t know why you are experiencing the issue that you are, but as a workaround you can add a shortcut under ‘Obsidian > Settings > Hotkeys > Use dark mode’ to make it relatively easy to switch dark mode on again without having to go into the settings each time.
Hope someone has a real solution for you.
Angel
Edit: See this thread, which says auto switching isn’t a thing, unless a plug-in or css snippet is being used.