This is the save I was using in the above video. I made this mistake again a few days ago triggering underground belt destruction. However I did not explicitly test it just now. I'll have time to in a few hours and will delete this post if I cannot reproduce it.
Thus, assume that it can be reproduced with this save despite the above disclaimer.
This looks like a failed upgrade, probably related to Construction Drones. The aboveground half is a yellow belt, but the underground half is a gray (Bob's basic) belt. I can add something to the next version to remove broken half-and-half connectors like this one.
Re: deconstruction picking up unintended underground belts
I'm still not able to get a deconstruction planner configured with item-on-ground to do anything unexpected, even with your gfycat.
There must be something you're doing that I'm not doing that isn't revealed by the video. How are you creating the deconstruction planner? Keyboard shortcut? Clicking the shortcut bar? Blueprint library? How are you configuring it?
So, the blueprint in question had been stored in toolbar 4. The blank space in this screenshot. I had just now removed it from my toolbar in to my inventory and now its gone for good on my local version of the save. Creating a new deconstruction planner as you had leads to the behavior you recorded in your video.
My assumption here is that it is some kind of data corruption issue and it was causing the same behavior that you fixed with the tree issue. At least if this blueprint was corrupted to also eat trees I would not know, as this save lacks any tree spawns.
If you still want to mess around with it, the deconstruction blueprint located in toolbar 4 in this slot (now empty in my screenshot because it vanished from all my autosaves, retroactively) should reproduce the issue if I am correct. https://i.imgur.com/MPAyL0G.png
Okay, so the deconstruction planner in question was in your blueprint library, and your save games had references to that planner, so when you removed the deconstruction planner from your library the references in all your saved games are removed, because the thing they were pointing to no longer exists. Since I don't have your blueprint library, I have the same thing in my copy of your save, a blank space. It might have disappeared from your library in the 0.18.x version where the whole blueprint library was overhauled.
I'm really curious how that planner in your library was configured, but at this point I don't see a way to find out anymore. :) I'm going to take another look at upgrade behavior with Construction Drones and see if I can figure out what happened with that halfway upgraded connector.