It's generally best to add garlic powder towards the end of sautéing aromatics. This prevents it from burning and developing a bitter taste.
Garlic powder, unlike fresh garlic, is very fine and concentrated. It contains sugars that caramelize and burn quickly when exposed to high heat for extended periods. When sautéing aromatics like onions and peppers, these ingredients typically require a longer cooking time to soften and release their flavors. If you add garlic powder at the beginning, it will likely burn before the other aromatics are ready.
To properly incorporate garlic powder, sauté your other aromatics first until they are nearly done. Then, add the garlic powder and cook for just a minute or two, stirring constantly, until fragrant. This short cooking time allows the garlic powder to bloom and release its flavor without burning.
If you accidentally burn your garlic powder, the dish can become bitter. To try and salvage it, immediately remove the pan from the heat and add a small amount of acid, like lemon juice or vinegar. This can sometimes help to neutralize the bitterness. However, in severe cases, it might be best to start over.