I’m extremely sceptical sucralose is responsible. It’s been subjected to a vast, vast amount of research to assess its safety and nothing to do with depression has come up as a side effect.
Though if I had to speculate, I’d say it’s something related to how your brain reacts to sweetness. How much sugar was in your previous diet? Sugar releases a large amount of endorphins when consumed, and these make you feel good. Going from a high-sugar diet to a low-sugar one (like Huel) would mean a lower rate of endorphins being released, which could manifest as lower mood or depressive symptoms. Additionally, it’s possible your brain associates sweetness with the endorphin release, and therefore downregulates its own production of these endorphins to account for those in your diet - this is actually similar to how heroin addiction works, though obviously a lot milder. As you’re then producing less of your own endorphins due to the taste of sweetness, but not getting having any released from the sucralose the same way sugar does, this could also be furthering your depression. If I’m right on that, it would start to improve after about a week.
All speculation of course, but something to think on.