Take a look at the diagram of Fogg behavior model here: Fogg Behavior Model | Behavior Design Lab. This is the model on which the Tiny Habits method is based. When you are tired, the activity of flossing and brushing becomes relatively hard to do. So along the X axis, you are close to 0. Your motivation also suffers when you are tired, so you are close to 0 on the Y axis, too. You are below the action line, where prompts fail.

Right now, you are engaged in fruitful behavior design where you figured out why on nights when you work you need to do flossing and brushing before going into work. It is very good that instead of beating yourself up, you are curious about the causes and opt for behavior design over self-chastising. That is the way to go!