If you have a task whose timestamp uses the .+x repeater, like:
** TODO Brush teeth DEADLINE: <2019-08-28 Wed 8:00 .+1d>
If the task is marked "DONE" at 2019-08-29 12:34, for instance, Emacs will set the new deadline to <2019-08-30 Fri 8:00 .+1d>, whereas Beorg sets it to <2019-08-30 Fri 12:34 .+1d>, changing the time of day. I think the time of day should not change, to align with Emacs's behavior in this case.
Thank you for reporting this. I've made a note to fix in a future release.