I have read a lot of questions on this site about this. Most do not have definite answers. I will try to set all the specifics!
I bought and planted a grafted avocado tree and it managed to survive. Most leaves fell, then new ones started to grow. I planted it in a 50L pot, more or less following the dimensions of the recommendations by the seller.
Conditions: I used a mix of perlite, some potting mix, and mostly coconut fiber, as it is what the seller recommended. I have only watered it twice, as it seems it does not need any more. In my climate and having more avocado plants, I have seen that most of the time once a month is enough (not in hot months). The first time it was a complete watering, the second one was aprox 1L, of water and a mix of fertilizer, with the proper amounts for 1L. Fertilizer contains:
Position: It gets daylight (Not direct sunlight). For a short time, it gets sunlight on the soil. I'd say 1 hour a day and not during the hottest hours.
Problem: The leaves of the avocado tree started browning on the tip and extremes more or less one month ago (see last picture). The measure for moisture says it's wet, the tree was growing new leaves properly and I do not think I overwatered it. Other avocado plants I have are having regular growth (in spurts as most avocado plants), and I water them taking into account the same moisture measurer for soil.
The only thing that "concerns" me is that another little avocado plant I have in the same position is also growing weirdly, whereas the other avocado plants seem fine.
This is the other small avocado in the same position, same conditions (never getting direct sunlight) which also is growing weirdly.
The following picture is the healthiest leaf, so the one that started browning latest.
The rest of the avocado plants (pruned most of them, but growing).
From everything I read (which would be long to cite) it could either be extremely dry and hot conditions (it's not hot, and I moisturise the environment, also having a sensor, it's never under 30%), it could also be salts/minerals from the water and/or the fertiliser, being too many, so I need to dilute it with more water. But is it smart to use more water when it does not need to be watered?
What other causes can it be? I kind of want to discard root rot because it had been growing fine and I did not water it, and the good growth kept for some time. I kind of discard pests as well, although I can never be sure.
PD: I added the browning tag, but have never seen it on this site even though there are a lot of questions regarding this phenomenon. Feel free to edit or delete it if it's not necessary!
Edit: This is a picture taken 1 month ago, 1st of June:
Edit2 (4 days after the question):
I called the place where I bought it, and told them what was happening. The first thing they told me is that the new leaves are probably not being able to cope with the strength of sunlight (it's getting quite hot). So to move it a bit away. After I told them that this couldn't be possible because it did not get direct sunlight, they told me that then the opposite might be happening, that it needs direct sunlight. I have a small growing light lamp in that room but the avocado tree is the farthest. I am not sure this is the problem though as the leaves started growing properly for some time...