The question is: is it likely that the reason I've been plateauing on my lifts that I'm not eating enough, given that my lean weight has also been stable or even decreased a little?

According to a couple of impedance tests, 2 months apart, I lost some 3 kg fat weight but apparently also lost 0.1 kg lean weight (which may be within the measurement error but still). I gained weight in total, but it is just more water.

That was not intended, I was trying to bulk or slow bulk. So I guess I need to eat more regardless.

I've also been mostly stuck on the same weights on the SL5x5 lifts, I was thinking it was because I go at times where I'm away from the gym and I compensate with body weight exercises (where I made some progress on "progressions").

Bonus question: fat % is calculated counting the water, which seems silly as it can vary a chunk just on how much extra water you have (and water is the most significant part of our total weight). Why not simply use lean/(lean+fat)?