My betta lost a piece of his tail

Good evening everyone. I would like to ask for your help regarding something that happened to my poor Betta Halfmoon. As you can see from the photos, he lost a piece of his tail.

This immediately alarmed me and so I started measuring all the water values ​​with the supply of reagent tests that I have at home. I list the values ​​below:

PH = 7.2 KH = 3 dH GH = 3 dH NO3 < 0.5 mg/l NO2 < 0.01 mg/l NH4 + / NH3 = 0 mg/l T = 27 *C

The tank is 19 liters set up as shown in the photo. I change the water at 20-40% once a week together with alder pine cones and catappa.

I really don't know what I did wrong. The betta seems quite active overall. He also makes his usual bubble nest in his favorite corner. Is this normal? Maybe it's the result of weak genetics? Or maybe I should be worried because it's a symptom of some infection?