Sadly none of those worked. BUT the good news I fixed it! If your using ELFX (Enhanced Lights and FX) Disable the dawn guard compatibility ESP within it. That seemed to do the trick!
I was having exactly the same problem. Disabling the mod fixed it.
Curious though. I had another problem with floors using the main ELFX mod. It affected wooden flooring with the exact same problem of objects dropping into a void and reappearing. Weirdly enough, this seemed to be a corruption problem linked to ModOrganizer. If I re-installed ELFX, the bug would go away for a number of sessions, then reappear randomly at a future date. And it wasn't just ELFX. I had texture problems with SMIM on the millhouse waterwheel, waterfall texture problems with SkyFalls+SkyMills, and food texture problems with another high-rez mod I installed. In each case, re-installing through MO fixed the problem temporarily. It wasn't until I got a recent patch update to MO that this re-corruption problem stopped. I haven't had to do a re-install of any of these for over a month now.
I tried re-installing the ELFX-Dawnguard patch from a fresh download of the plugin, however that didn't fix the issue like it did with the others. Removing the plugin, however, seems to have done the trick. Hopefully the Dragonborn plugin isn't similarly affected.