This one is reproducible on the demo site today and that’s where this screenshot is from. Begin to create a new event. Set to recurring Monthly. Note that below “Never” is checked and the “10/11/2020” box is disabled. Now go and change your start date to any other date above. This box enables itself. It should not do this because the ending date option is not actually selected.

I fixed this in JavaScript by hooking into some events rather than modifying the Kendo.all.js file.