But...there is a solution that works (although it may be a bit confusing at first). A toggle with two buttons, dark and light. Both cannot be pressed at the same time, but both can be unpressed at the same time. That's the auto mode, and the ui can be tweaked to show a little label in between or below.

In auto, none of the buttons is pressed (and preferable a small label that says auto is shown). Pressing dark/light will force that mode, enable that button and remove the auto label.

In dark/light mode, you can press the other button to enable the other state. Or you can press the current button to disable it, and switch to auto. This last one is the confusing-at-first interaction, but makes sense if you think about it (it's like saying: I want dark / I want light / I don't care)