This is a really weird issue to even describe. My lab blueprint exhibits a strange behavior when building it over top an existing instance of itself, where all the belts get flipped across, thereby swapping their lanes. I've provided this blueprint book to reproduce the issue, which is more or less my lab setup stripped down to only the belts:
So to reproduce it, get the first blueprint in the book that includes the infinity chests and such, flip it horizontally and place it. Let the belt fill up with items, then get the second blueprint, flip it as well, and align and place it over the existing belts. You should observe that the lanes of science packs get swapped.
Here's a video as well to show the behavior: https://qimg.techjargaming.com/v/3J9ZCz8B.mp4
I am on a modded save, but I doubt my mods would have any effect on this. Still if you are unable to reproduce it I will investigate further.
Edit: I have updated the thread title as the issue seems to be more generalized than my original testing.
Ref.: 134420 134649
[boskid][2.1.7] Pasting belts over existing belts can flip lanes
[boskid][2.1.7] Pasting belts over existing belts can flip lanes
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Last edited by Techjar on Sun Jun 28, 2026 6:08 pm, edited 4 times in total.
Re: [2.1.7] Building specific flipped blueprint over itself repeatedly flips belts on perpendicular axis
I investigated a bit and the behavior seems to be tied to underground belts.
Re: [boskid][2.1.7] Pasting belts over existing belts can flip lanes
Thanks for the report. Issue is now fixed for 2.1.10.