It would still be a (albeit convenient for google) bug, as the setting indicates closing all windows as the trigger for deletion.

It's convenient that "zombie processes" only retain data for Google-owned sites https://news.ycombinator.com/item?id=49583517

At least on windows, closing chrome doesn't close the system tray process which keeps Chrome and Gemini prestaged and cached. Wonder if OP is experiencing a version of that on his Mac?