I was manually constructing rows of blue belts when they started randomly placing undergrounds for no reason. I don't have the slightest clue why this could be happening, it also sometimes doesn't remove the regular belts in between the undergrounds.
I was placing these belts really quickly so that might have something to do with it, I also haven't had this issue with any other type of belt other than blue.
This is my first post, sorry if I left anything important out.