Carl-- MG soils are very often food amended, and too hot initially for seedlings; I found conflicting info on this one; one site it's rich enough to support 2 months growth, another say no nutes added-

.. Mass produced soils are shaky for quality consistency as well; MG is notorious for problems that I've seen here,... have you checked the soil pH? If you're using RO water, and pH'ing it to 6.5, it's not going to stay there long! RO has no pH buffering capacity, and will swing wildly from small inputs and be very unstable,... I'm thinking the soil is too acidic at this point, and feeds being acidic too, will worsen matters,.. Dolomite is fine, but it's slow acting, and if it's too course grained it'll be
really slow acting, to release mineral nutes and buffer pH,...
.. how old is this seedling? There's several things going wrong here, clearly not liking that soil unfortunately,... A pH reading is critical, to see if it's a lock-out or plain lack-of nute's issue. do a run-off test--(
https://www.autoflower.org/threads/...-testing-and-ph-estimation.41733/#post-928975 ) .... try the soil right from the bag first to see where your starting point was,...