What you are starting out making is first a water-in-oil emulsion. There’s nothing wrong with that, and lecithin alone might work to stabilize it. But as you keep adding water, the lecithin/oil interface between the water droplets gets thinner and thinner, and at some point the interfaces start to break. A water-in-oil emulsion can only stand a little bit of this before it inverts, and becomes an oil-in-water emulsion, which is exactly what’s going on here. Since lecithin is completely unable to sustain an oil-in-water emulsion by itself, your whole batch falls apart.

To keep your formula stable, you are going to need to add at least one, and preferably two or more, oil-in-water emulsifiers. You might even want to look into emulsifiers that will let you make a micro-emulsion (really tiny oil droplets), which will be more fluid and yet possibly more stable than an ordinary macro-emulsion.