When using the upgrade planner, if you choose to upgrade blue undergrounds to the purple, the bots go to "pickup" the blue ones, but do not actually remove them. This results in the bots infinitely filling up chests with blue underground belts.
Tried now and work correcly with only K2, so, or u use a mod that modify belts or this is nothing that concern mods, and is another bug of 1.1. Please if u think is not caused by mods, post this on the forum to make able the dev to fix it: https://forums.factorio.com/viewforum.php?f=7
