Thanks for bringing this to my attention. I haven't covered this case yet where the timestamp appears in the headline rather than the planning line below. I'll look to see how I could cover this but may need to wait until a planned (although not scheduled) rewrite of the parsing/writing of org files in Swift.
Aaron Zeng
Given the following task:
Beorg, upon marking this item done, adds a new plain timestamp as the first line of the entry, rather than modifying the timestamp in the headline as org-mode does.