My custom Walker Nav Menu Edit override doesn't save just menu orders

I’m developing my theme and I had overridden the nav_walker_menu & walker_nav_menu_edit files. And then my custom walker_EDIT works fine, but from some time it doesn’t save the menu order. The other options for menu are saved with no problem, but only the menu order is not. Even menu and sub-menu structure(hierarchy) is saved normally.

I tried all the way that I found about the related problems and answers, but that’s neither a php_var problem nor a browser issue. The number of menu items has nothing to do with this problem. I’ve also tested it with 10 menu items but it still doesn’t work.

Please help!

p.s: It’s not the plugins. Because I had swtiched to another theme, then it works! The problem is just in my theme.