more than one thing can cause tip burn... and a little tip burn that doesn't progress isn't anything to worry about. So, i'd wait and see on this one.
you have a lot of wildcards between soil and amendments. keep track of what you add - aggregate ratio and concentration of each nute. it will help eliminate possibilities later, if needed.
even if we choose to do the wrong thing, we can use that info retroactively to make a better decision next time. eventually you'll smooth it out. if you keep making the soil in the same way each time it'll have some consistency.