I've had this issue for a number of updates now.
If I insert a check box at the beginning of a paragraph and then hit a hard return in the same paragraph it will delete the last character in the paragraph.
Each additional hard return deletes an additional character.
My workaround is to hit all hard returns first, then add individual check boxes to each line.
It was better in the past, where I could take a single paragraph, add a single check box, and then hit multiple hard returns to automatically add check boxes.
The workaround requires repeated use of cmd+shift+T,