What couse this and how do I prevent it

This is 7 day old seedling.. Automatic

Today I transplant it in 20l pot because root tips start popping from bottom of 0.5l pot where I plant a seed... Some of the roots start even change color to brown and I know that is not good so I transplant it..
So can that be the reason for this rust on 2 fan leaves or something else is the problem

Looks like high P or K. Looks like it's already on the mend.

Yes definitely this soil can be too hot with p or k...
So will it be big problem for future growth?

Can you please explain this... it's already on the mend?

Thank you

Won't be a problem, just a flash burn.

It's already on the mend, it's adjusted and not spread to the newest tips or any further back the leaves.

Should be good to grow.