Steps to reproduce
In a list made before the checkbox change, or with a text copied from a list made before the checkbox change, with “Show checkbox in front of checklist parent” turned OFF, toggle Make checklist on a parent. In the example below I do the toggle on “Ute”.
All the 2nd and 3rd level items should get a checkbox.
Only the items with sub-items and their respective sub-items get a checkbox:
Firefox 69.0a1 nightly or Chrome 75.0.3770.100 on Windows 10.
If I turn “Show checkbox in front of checklist parent” ON, while the list is in the “made checklist” state, all items get checkboxes as expected. But if I then do “unmake checklist” on “Ute” again I get this: