I’ve noticed that recently too, but I think it may be intentional and I actually prefer it this way. The * or - lets you indent your checklist items into structured lists without the added noise of the bullet point.
- [] Milestone 1
- [] task 1
- [] task 2
- [] task 3
- [] Milestone 2
- [] task 1
- [] task 2
- [] task 3
- Milestone 1
- task 1
- task 2
- task 3
- Milestone 2
- task 1
- task 2
- task 3
vs
[] Milestone 1
[] task 1
[] task 2
[] task 3
[] Milestone 2
[] task 1
[] task 2
[] task 3
Milestone 1
task 1
task 2
task 3
Milestone 2
task 1
task 2
task 3