Holy crap, I hope this is an isolated incident. I haven't yet encountered this, only spawns being deleted when saving with another person over XBL. I wish I had some helpful advice for you, sir. If you figure out how to make it work, please let us know.
one thing i do remember - i didn't use a canvas, but instead deleted all the building materials from the default map and dropped the game-play items down the shaft - maybe they didn't survive the drop - some other items like territory markers vanished as well
Tripmines detonate on impact after falling down the crypt tunnel, so they may have merged with the map geometry somehow. I love the way Mastar deleted all those posts just so he could reserve for pics, but look how much stuff has been posted in the reserved posts...
I've noticed you can move small objects with large objects, but you cant move them while your in person form, like a dumpster