leaf tips are dying back, what is the cause for this?

I also really like your use of great white myco in the mix, that stuff is fire🔥🔥🔥

Plants are looking to be getting a bit to much feed at this stage of life. The extra dark geen follage is a sign its to much N. If you hold off and just water it will correct its self. If the medium has a buildup in it a small flush will be needed but flushing soil is harder as it more prone to compacting and getting water logged affecting the roots. Its needs to be done low and slow not just a giant jug of water tossed in. Given you are growing in Soil it is much more absorbent of liquid nutrients, I would recommend only feeding once a week and at 1/2 or 1/3 the amount suggested. It would also be a good to lower the concentration using the EC pen as a monitor. Mix up the feeding solution then dilute it down with more water till you hit your desired point. I would not be going over 1.3 at this moment and drop it down to 0.8 for a bit. If there is any runoff I would collect it and check the EC and PH of that solution as well. It will be telling of whats happening in the medium. Best Of Luck!

Your doing everything perfectly man don't stress out even pros have yellow tips on some of their leafs just nutrient burn id cut back 15% on nutrients

The tips turn brown because of nutrient burn. You use too much nutritions. This is just te very beginning from a nutrient burn so good that you noticed it early! To solve this problem just cut back the nutrients to 75% from your last given dose. The tips that are brown will stay brown and won't recover. But when you cut back to 3/4 of the previous given dose the rest of the plant will stay green and the plant will not be harmed further by overfertilizing.