It's documented in HELP MAP (which HELP SUBDIVISIONS points you to), but
you -cannot- leave things outside in subdivisions if you want them to be
there when you get back. Subdivision rooms are not 'real' insofar as
they disappear soon after there are no players in them (just like any
room that uses the overhead map system). When they disappear, everything
in them is destroyed along with them. It's the only way we can do
overhead map-style systems without running into massive database bloat.
We're working on a work-around for this in subdivisions (personal items
like mounts would be transported to a certain room in a subdivision
instead of being destroyed), but just be aware that until this happens,
leaving your mount (or whatever) outside in a subdivision will get it
destroyed. Rooms inside houses, or the room that a house is built on,
are safe to leave your stuff in.

Penned by my hand on the 11th of Aeguary, in the year 327 AF.