Long story short. Why when I delete a node in Inkscape it becomes a straight line? It used to be automatically curved. I miss this function very much.
I was about to post an image that explains my problem but just now I have checked and the problem has been fixed somehow. I just played with settings on the horizontal menu and in "Preferences". At first, nothing changed and I lost hope but when I checked now it works as expected. I must have tweaked some settings for the problem to appear. Anyway, I would like to know what was that for the future.
P.S. Thank you for responding, everyone. :)