I guess that this is because with Markdown the item number has no real meaning apart from indicating what follows is part of a numbered list. That said, the number used for the very first list itemdoes have significance as the renderer will count up from that number.
It is when the Markdown is rendered that auto-numbering is applied.
I agree that list numbering has special and limited meaning in MD format, still as far as I use MD to WRITE and READ notes (I believe that's the main purpose of the app) I'd like to see a correct order of numbers. I don't like dual panel mode because it eats space and not really useful for me.
As a resume, Joplin doesn't have this option neither automatically nor manually with hotkey. Am I correct?
Up until the rich-text editor Joplin has used the method where markdown is used to create the notes but the main way of viewing them is using the render pane.
Not that I am aware of. A person who has done a lot of work with the CodeMirror editor is @CalebJohn. He has created a plugin to enhance the Markdown editor so the render pane is no longer required. Maybe he can advise you as to whether what you are looking for is possible to implement.
I think you will want to try the markdown prettier. It adds a keybinding that will autoformat your markdown notes (i think it supports re-ordering lists).