Hey Shoepatcher!, thanks for your answer!, here is the front and backsides. When the knot gets at the bottom, and even though I just change the upper tension the slightests, the knots get on top. Even trying one several meters of leather, I cannot find the sweet spot.
The leather is 3.5mm thickness.
Needle is SCMETZ NM; 130 SIZE: 21 134 LR
Thread is is Serafil 20 Tex 125
Bobbin tension: I do the drop test. where I drop the bobbin like a yoyo, and the thread gives a little.