I don't grow in soil but to me looks like a ph problem drop by these fellas https://www.autoflower.org/threads/afns-live-help-thread-replies-by-afn-staff-only.30723/page-625 see if they can help or @A-Train think he grows in the same medium hope you sort it out
I just don't understand how if they're from the same batch of ProMix, and they're being fed from the same mix of nutrients. Unless its from the first 10 days of my pH'ing my water.