Why is my unsweetened vanilla almond milk sometimes thicker and creamier and when I first get it, it tastes thin, like water?

Before pouring it in, I guess you did not shake it. Or not enough to shake?

It varies between brands. Some of them have more thickeners and stabilisers and will not separate them, but they usually have to be shaken with nut milk to spread nut particles.

If you think that your milk is thin when you first start drinking and it gets thicker in the bottom, it would make sense to drink the thinner, watering part at the top and more focused stuff at the bottom of the cardboard if you don’t shake each time.