What did you do?
I wanted to place down a new agricultural tower. This needed soil below it to grow crops, but there was a mix of what could be used. Overgrowth had to be used for most of it, but some could use the more basic soil, which was already placed down in areas it could be used. As I still needed to place the overgrowth though, I increased the size of the brush to be the same size as the agricultural tower's coverage, and placed down overgrowth soil, then I quickly (before the bots delivered all of the soil) switched to the basic, artificial soil, and clicked again to hopefully cancel the overgrowth soil in areas where artificial soil could be used.
What happened?
The bots still tried to deliver the overgrowth soil, and then got stuck in mid-air over the areas the order had been canceled.
What did you expect to happen instead?
The bots to not try to deliver the overgrowth soil any more and go back to doing other things.
Does it happen always, once, or sometimes?
I'm able to always repeat the issue following the steps I described in "What did I do".
Edit: Looking at it more closely, I think they're actually trying to deliver artificial soil and not overgrowth, but that soil was already there. So somehow queuing an order to replace artificial with overgrowth, then overwriting parts of that order with artificial (that was already placed hours earlier), causes the bots to try to overwrite artificial soil with artificial soil, which I assume is why they got stuck?