fix your watering habits, they may not be the cause now, but will eventualyl cause something.
water entier substrate.. never a portion of it. That only leaves potential dry pockets and that increases risk of toxic buildup.
water entire pot, large or small. Wait until top layer dries before repteating same process. If you do this at same loss of weight, it'll require same volume of water -- a volume dictated by the physical properties of your substrate and not a number you choose out of the ether.. not any uninformed person's suggestion.
teh type of substrate will impact the trigger point. e..g coco holds less water per volume, all other facotrs te same, so you don't wait for it to dry as deeply. Where as soil typically holds a lot more water per volume and you want for top 1" deep to dry before you irrigate again. Some nuance for a very tiny pot for seedlings.. obviously 1" deep could be half the damn pot and that would be a problem. So, some common sense needs to be applied to this generalization. Cuttings need a slightly different approachs early on. With no roots the moisture in substrate can't ebb and flow as much without a negative effect occuring.
This is probably environmental or slight over feed.