I have plenty of these beauties, as I made more last year from cuttings of about five parent plants. The parent plants are fine, but they are in a different area of the garden. I have two in particular that have been turning yellow. They are planted in a well-drained soil with a lot of hardwood mulch on top (no crown rot). Someone told me they like the soil to be a bit alkaline, but my others (parents) are planted similarly. However, the parent plants' soil is more wet, as it is sand and clay. I tried a couple handfulls of agricultural lime, but it did not do anything. They also do not appear to be infested with any visable mite/paracite. Oh, I also bought the Home Depot version of the 'Russian cypress' and they are planted the same way as the real Microbiota decussatas that are yellowing, but the Home Depot ones are fine, just a different texture (probably not the M. d. species).
