Getting sub tasks onto timeline

If you want to use subtasks, at this time, you may have to live with some unwanted behavior like what you mention here.

There is a topic where you can vote for adding subtasks to timelines.

There are many users that use subtasks. But I personally find they currently come with more cons than pros.

I recommend you search the forum for subtasks and you will find a number of conversations on the subject and decide for yourself the best way to organize your Projects.