When you have an item with a DEADLINE that includes both a repeater and a special warning period, such as DEADLINE: <2018-12-31 Mon 09:00 +1w -1d>, it seems to not be detected as a timestamp. Such items will not appear in the agenda view at all, and will appear in the TODO view, when you filter by "No Dates".
Currently the special warning periods are not supported. A repeater however should be recognized and used for displaying tasks in the agenda.
The next update to beorg will handle a repeater in that the state will be reset to TODO (i.e. the first todo action state, or the value of org-todo-repeat-to-state) and the date correctly updated based on the repeater type. There will also be a UI to create and modify repeaters for dates.
In response to the above messages, my report here is not about repeater functionality. It's about timestamps that have a special warning period not being recognized as timestamps at all.
Likewise, timestamps such as <2019-01-07 Mon +30d/90d>, typically used with org-habit, are not recognized. Items scheduled in this way don't show in the agenda view, and do show in the TODO view with the "No dates" filter, contrary to user expectation.
I think it would be important to first have these recognized as just regular timestamps, even if ignoring any special warning periods, habit windows or repeaters, just so that any existing timestamps that I have created via emacs are interpreted correctly on beorg.
I'll look at adding support (in addition to special warning periods) for org-habit like timestamps in a future update. Is anyone aware of any additional timestamp formats for which it would be useful to also have support?