Why doesn't it show any deficiencies? While my autoflowering plants in the same conditions immediately turn yellow?

Because it’s enough soil for that little plant. Keep going, you’re doing a good job

Light mix 1.2ms/cm. As the organic stuff breaks down to constantly releases more salt ions that contribute to "EC". So it might only be 1.2ms/cm, but it could have 150ms/cm stored up in "organic matter" that has yet to be broken down. When you get the microbes eating carbon, breathing oxygen, giving off CO2, those aerobic soil microbes will carry about 0.5V of electricity that makes up the EC. Very similar to chelation. The microorganisms will take a metal-based mineral and a non-metal-based mineral with about 1000 different combinations, and they will create an organic salt! That doesn't kill them, that the plant loves, that the plant enjoys. This creates an environment that is conducive to growing its own food. Keep the pH between 6.2 and 6.6, and the plant will be able to cycle the medium's CEC for its needs using hydrogen.

Why is this a problem? You should be thankful!