Apologies, seen micro grow and thought you were using small pots. PH is way off for a soilless grow, that is probably your issue as your still adding in nitrogen (grow nutes) which is the only other reason i can think of yellowing leaves.
@Will_journals, Honestly ive really neglected the entire PH factor for this grow, I use the tap as is, Ill definitely invest in a ph meter + ph adjusters for the next grow.
Root bounding, recently experienced this with an autoflower of my own. That's why I grow in fabric bags, can easily drop that bag into a bigger one...easy mess free transplant.