Homemade soy milk powder, or other plant-based milk powder

You are going about the process the wrong way - the milk powder in this case is made of pulverized soy beans. Once you add water to this powder to make the milk, it will be very difficult, time consuming and expensive to dehydrate it to make the powder.

The reason your dehydrating milk is turning sour is either that it has picked up some bacteria or yeast from the air or contact with a slightly contaminated surface and these are growing happily in the nice warm dehydrator, similar to making yoghurt from milk. The other possibility is that there are some (bio)chemical changes going on with the heating which result in a sour taste. I would put my money on the bacteria/yeast option.

Your best bet will be to buy some dry soy beans, soak them to remove any dirt, then dry overnight, roast to denature proteins then pulverize them yourself. You can use a blender/spice grinder to do much of the work, but to get it fine enough to work well as milk, you might need to do a bit of manual labour with a mortar and pestle. There is a recipe here.