Issues with AnimationCurves

Hello everybody,

I have a class (scriptableobject) and for that class I have a editor (custom inspector) script. The issue that I am having is that whenever I try to edit the animationcurve property, the animationcurve simply resets/clears out. For example whenever I press on the preset the curve appears for half a second and then disappears. I thought it was a bug, but I tried to make some simple similar script within other project and I didn't had this issue. There's no errors in the console, I apply all the modifies property changes at the very end and update the serializedobject at the very start of the method. So I'm not sure what I'm missing..

Can someone give some advise? Thanks