The syntax for checklists is ‘- [ ]’, so an unordered list. You may be able to type a checkbox in a numbered list, but I think the behavior you’re seeing is because Obsidian still sees your numbered list as a numbered list and not as a task list. Does this behavior also occur if you change to an unordered task list and then indent one of the tasks?
Has indenting an ordered task list worked before? If so, what you’re seeing may be a bug in either Obsidian or a plugin you’re using. If not (I can’t get it to work in my vault but I’ve never tried before) then it’s probably Obsidian not supporting numbered task lists.